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Make multiplatform application for mac os from windows environment using WPF .Net.Core [duplicate]

Microsoft announced .NET Core 3 comes with WPF and Windows Forms. So can I create a desktop application for Linux or Mac using .NET Core 3?

No, they have clearly stated that these are windows only. In one of the .NET Core 3.0 discussions, they have also clarified that they do not intend to make these features cross-platform in the future since the whole concept is derived from windows specific features. They talked about thinking of a whole new idea for cross-platform applications, which is not easy.
Source: https://youtu.be/HNLZQeu05BY
Update
The newly announced .NET 5 now aims in avoiding all this confusion by no longer calling it ".NET Core".
Update 2
With blazor client-side (releases on may, 2020), there is a new experimental project for cross-platform apps using webview that is in the works.
Source:
https://blog.stevensanderson.com/2019/11/01/exploring-lighter-alternatives-to-electron-for-hosting-a-blazor-desktop-app/

No with WPF or Winforms. But you still can develop desktop apps on Linux, with .NET Core, using other libs like GtkSharp or Avalonia

Well, the name itself says Windows Presentation Foundation. So it's primarily windows based.
But it is worth to be noted that the new UI framework, .NET MAUI that will be shipped with .NET 6 is cross-platform, with a single-codebase, single-project structure.
Currently in Preview at the time of writing, to be shipped in November 2021. Here's the roadmap.

Although it is not native dotnet core on linux, but it may be helpful - using wine.
I find a very useful comment in the discussion here, more details here
In other words, it is possible to use windows version of dotnet core under wine.
I didn't try it yet, but from the article I mentioned I can say it looks nice and might be an option.

What’s the difference between NETStandard.Library package and Microsoft.NETCore.App packages?

I wanted to know the difference between the two packages and if it had anything to do with .NETCore and NETStandard and compatibility. I found this:
The NETStandard target framework is an abstract target framework that represents API surface of many frameworks and platforms. As such NETStandard assemblies can run on any platform that supports the NETStandard targeted by that assembly, for example: .NET Desktop, Windows Phone, Universal Windows Platform applications, .NET Core applications, etc. NETCoreApplication is a concrete target framework that represents a single platform with both API surface and implementation. .NET Core applications are runnable on their own. .NETStandard libraries must be published-for or consumed-by a specific concrete target framework to be used in that type of application.
on https://masteringalm.github.io/framework/2018/07/24/NetStandardvsNetCoreApp-Project-Types.html
But I’m not really sure how to use this to describe the differences between the two packages. I saw a post on this link: What's difference between .NetCoreApp and .NetStandard.Library?
Where it said:
NetCoreApp is a platform and .NetStandard.Library is a library supposed to be cross platform (portable class library) for various .NET platforms runtimes.
I think this connects to differences between the two specific packages mentioned in the question header, but could anyone describe the differences between the packages in a not so convoluted way?

.net standard
Each implementation of the managed framework has its own set of Base Class Libraries. The Base Class Library (BCL) contains classes such as exception handling, strings, XML, I/O, networking, and collections.
.NET Standard is a specification for implementing the BCL. Since a .NET implementation is required to follow this standard, application developers will not have to worry about different versions of the BCL for each managed framework implementation.
Framework Class Libraries (FCL) such as WPF, WCF, and ASP.NET are not part of the BCL, and therefore are not included in .NET Standard.
The relationship between .NET Standard and a .NET implementation is the same as between the HTML specification and a browser. The second is an implementation of the first.
Hence, the .NET Framework, Xamarin, and .NET Core each implement .NET Standard for the BCL in their managed framework. Since the computer industry will continue to introduce new hardware and operating systems, there will be new managed frameworks for .NET. This standard allows application developers to know that there will be a consistent set of APIs that they can rely on.
Each .NET version has an associated version of the .NET Standard.
.NET Core is a free, cross-platform, open source implementation of the managed framework. It supports four types of applications: console, ASP.NET Core, cloud, and Universal Windows Platform (UWP). Windows Forms and Windows Presentation Foundation(WPF) are not part of .NET Core.
Technically, .NET Core only supports console applications. ASP.NET Core and UWP are application models built on top of .NET Core.
Unlike the .NET Framework, .NET Core is not considered a Windows component. Therefore, updates come as NuGet packages, not through Windows Update. Since the .NET Core runtime is installed App-Local, and applications are updated through the package manager, applications can be associated with a particular .NET Core version and be updated individually.
For more information visit https://www.infoq.com/news/2017/10/dotnet-core-standard-difference/

Overly simplistic, but think of .NET Standard as a library that runs on any platform that can run .NET. This means it can run on Linux, Mac, Windows, etc. .NET Core is a minimalistic version of the "framework", so you can keep your your apps as small as possible and only add what you need. Core means only the core functionality that is absolutely required for an app to run.
Why is standard and core different? It is largely because the functionality of .NET on other platforms started open source with someone other than Microsoft. As such, the features have lagged behind .NET. The name of the .NET "framework" on other platforms was Mono. As some point .NET Core may well replace standard, but certain things have to get to the same level.
I imagine some purist will disagree with my oversimplification, but it is an easy way to get your head around the libraries.
As for what specifically is different between the two (or "what is missing from standard to be equivalent to core") - I am not sure. I am sure it can be looked up. In many cases, I have found Core works fine, such as dockerizing the application (Yeah, I made the word "dockerizing", but meaning running in a Docker container). The problems are when you run some package that cannot be handled in "Mono".
When it comes to packages you add into the standard or Core (Nuget anybody?), where there is a difference deals with one having features that cannot run in standard.

Is there a way to use UWP dll in normal .NET Framework project?

New to UWP. I like the idea that it is difficult (or impossible) to decompile UWP application because it is built on the .NET Native/Core... I'm wondering if is possible/a way to use UWP dll in normal .NET Framework?
I thought perhaps I could code my logic and stuff in UWP dll instead and leave all the UI stuff intact in my .NET application... so far everything I read online are the other way around, people try to reuse their existing .NET Framework libraries in UWP.

You can write a DLL targeting .Net Standard version 1.0 all the way up to 2.0 (which is the max that UWP supports in the Fall Creators Update). Once your DLL is ready, you can use it in UWP and .Net framework.
Look at the chart in this page https://learn.microsoft.com/en-us/dotnet/standard/net-standard it will help you decide which .Net Standard version to target and shows you the compatibility with various other .Net tech.
This is what I do now when I need to share code between UWP, WPF, ASP.NET, and other types of projects.
Be advised that sometimes this is not very easy, as it might require you to make some changes to your code in order for it to be compatible with .Net Standard.
I strongly suggest you look into .Net Standard as it will help you a lot.

What are the platforms in the .NET Platform Standard?

Currently trying to learn about the .NET Platform Standard I've found myself quite confused about the idea of "different platforms".
I'll try to make my point clear. What I currently now about the .NET Framework is that .NET is roughly speaking made up of the CLR, the BCL and supporting software to boot the CLR and provide the interface between the virtual machine and the underlying OS.
So when we code using the .NET Framework we indeed target some version of the framework because the types we are using from the BCL come with the framework and so depend on the specific version.
Now, .NET Core is quite different as I understood. It is not all packed together like that. We have the CoreCLR which is a lightweight VM to run the IL, the CoreFX which are the libraries properly organized as NuGet packages and we had up to now the DNX/DNVM/DNU which provided the supporting stuff like booting the CoreCLR and interfacing with the OS.
Anyway, despite if we install the framework on Windows 7, Windows 8 or Windows 10, we code against the framework.
Now, on the .NET Platform Standard spec we see the following definition:
Platform - e.g. .NET Framework 4.5, .NET Framework 4.6, Windows Phone 8.1, MonoTouch, UWP, etc.
Also we see after that a list of platforms, which includes
.NET Framework 2.0 - 4.6
Windows 8
Windows Phone 8.1
Silverlight 4, 5
DNX on .NET Framework 4.5.1 - 4.6
DNX on .NET Core 5.0
Now this confuses me completely. I always though: we code against the .NET Framework and the framework is the framework no matter what.
But here we have these platforms which includes the .NET framework as just one of many platforms. We have for example Windows 8, but wait a minute, running .NET on Windows 8 is not just the same thing as running .NET on any other OS? Why it is separate from the .NET Framework 2.0 - 4.6 platform?
We also have DNX as a specific platform. This makes me wonder: the platform is that "supporting stuff" related to booting the Virtual Machine and providing the interface with the OS? Or the platform includes the Virtual Machine?
Anyway, as can be seen I'm quite confused. What are those platforms indeed and how this relates to my current understanding of the .NET Framework? Also, why .NET Framework 2.0 - 4.6 is described separetely? Isn't everything described here some version of .NET Framework unless .NET Core?

we code against the framework.
Well, sure you are. When you manipulate strings in your code then you'll always use System.String. And it (almost) always behaves the exact same way with the exact same methods and properties.
But displaying the string does have implementation details that you cannot really ignore:
If you want to show it in a Unix terminal on Linux or OSX then you need to target Mono or CoreCLR, the framework implementations that can run on such operating systems.
If you want to show it in a Windows Store app (aka WinRT, aka Windows 8, aka UWP) then it is actually a HSTRING under the hood, an very well hidden detail that you don't have to worry about. But does require an UI gadget, like Windows.UI.Xaml.Controls.TextBlock, a class that is highly specific to WinRT
If you want to show it in a browser then you need to target ASP.NET or Silverlight, framework hosts that were optimized to run on a web server or as an add-in for a browser.
If you want to show it on a device that is powered by a small lithium-ion battery, like a phone, then you'll inevitably have to deal with a framework version that was optimized to use as little power as possible. That does affect the code you have to write, there is a huge difference between code that burns 100 Watts and code that keeps a tiny battery alive for 8 hours. Nothing you can directly see, beyond the need to use async/await a lot, but certainly something that affected the runtime very heavily. Targeting Xamarin or WinRT is required.
If you want to show it on any operating system then you do need to target a framework version that does not use the kind of tricks that .NET uses on Windows to have an EXE launch the CLR virtual machine. That requires dnx.exe, just like you'd use java.exe or python.exe for programs written in Java or Python.
It would be lovely if those implementation details did not matter. But not the way it works in practice, as .NET proliferates and becomes available on more and more devices and operating systems it inevitably also becomes more convoluted. Choose your intended targets early, it is important.

There are many Frameworks (.NET Framework, WinRT, UWP, Silverlight, .NET Core, Windows Phone, Mono, Micro Framework und the old Compact Framework) not just only the .NET Framework.
The new way is to program against a platform standard which supports one or more of this frameworks. The platform standard defines an API which matches one or more frameworks. This means if your application supports platform standard 1.1 you will probably support almost all frameworks. Platform standard 1.4 will support .NET Framework 4.6.x and .NET Core only
Have a look at this document: https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md

Now this confuses me completely. I always though: we code against the .NET Framework and the framework is the framework no matter what.
No, there are actually plural .NET frameworks or platforms as you like to call them. Prior to .NET Standard, you used to target a single framework (maybe the full one, currently at version 4.6.3, if you develop web applications or windows services). DLLs targeting a framework are not compatible with another one. I.E. a DLL developed for the full .NET framework cannot be executed on Windows phone 8.1.
These frameworks actually implement a quite small common set of librairies, but also specific libraries for dealing with the platform they are intended for. I.E. libraries for managing a web server hosted on IIS in the full .NET framework, or functions for dealing with a mobile phone in the windows phone 8.1 framework.
Before .NET Standard was the PCL
There was although a workaround, called PCL which stands for "Portable Class Libraries". By using only the small common subset of methods/assemblies in two or more .NET frameworks, one could develop a library that could be included in projects targeting different frameworks. For instance, PCL profile 37 means you want your library to be usable in .NET Framework 4, Silverlight 5, and Windows 8 projects.
Please have a look at this for a list of PCL profiles and their compatibilities (I don't know if it's exhaustive): http://danrigby.com/2014/05/14/supported-pcl-profiles-xamarin-for-visual-studio-2/
Now what about .NET Standard ?
The goal with .NET Standard is to simplify this and get rid of PCLs. Roughly, .NET Standard defines a contract (a set of classes and methods), that will be implemented by all .NET frameworks. If you develop a library that targets .NET Standard, you're sure it can run on all .NET frameworks. It's the basic idea/goal behind it (even though it's a bit more subtle).
Have a look at this for the exact compatibility: https://blogs.msdn.microsoft.com/dotnet/2016/09/26/introducing-net-standard/#user-content-whats-new-in-net-standard-20
If you look at the compatibility table, you'll see that a library targeting .NET Standard 1.6 is usable as is (no need to recompile it) in .NET Framework 4.6.3 and .NET Core 1.0 applications.
In other words, we can say that .NET Framework 4.6.3 and .NET Core 1.0 both implement the .NET Standard 1.6 contract: its classes and methods.
If you also want your DLL to be usable in a windows phone 8.1 project, you'll have to target .NET Standard 1.2, which offers less functions than .NET Standard 1.6 (no System.Net.Sockets for instance).
See here for a list of available namespaces in each version of .NET Standard https://github.com/dotnet/corefx/blob/master/Documentation/architecture/net-platform-standard.md#user-content-list-of-net-corefx-apis-and-their-associated-net-platform-standard-version

Can PCL libraries run with ASP.NET MVC?

I wish to write a library that can run on a number of targets such as WPF, Windows Phone/Mobile, Mono, and ASP.NET. I keep searching for information on this but it seems PCL's are not supported in ASP.NET?
Even VS indicates PCL can't be used with ASP.NET:
So what should I do to be able to write a PCL that runs on all the platforms? Use PCL for everything apart from ASP.NET, and create a normal Class Library for ASP.NET and basically copy+paste the code?

The short answer is YES, as long as the PCL profile supports the same .NET framework version your MVC application is targeting, you will be fine.
When you create a Portable Class Library, you can choose a combination of platforms that you want your code to run on. The compatibility choices you make when creating a Portable Class Library are translated into a "Profile” identifier, which describes which platforms the library supports.
What probably confused you, was the fact that ASP.NET was not mentioned in the list of supported software frameworks (platforms). The reason is that ASP.NET is not really a software framework, but an application framework (higher level). This means that whatever application frameworks target the software framework the PCL supports, the application frameworks will also work.
Be sure to take extra caution when choosing the supported platforms when creating a new PCL project, since the API that is available for use in that project depends on that choice!
You can read more about portable class libraries on MSDN.