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C# and ASP.NET Core 6 : authentication and user details in "session"

I'm going to get so many "okay grandpa" comments for this one.
I've read a dozen articles and every SO question I could find on this subject.
I must have been away too long or missed something completely, because I swear that user authentication used to be very simple. I seem to recall built-in methods and a session on the server simply knowing who the user was via a cookie or similar, with the ability to store information "in session". I don't recall even setting up authentication in years past, it was just built-in to new applications.
Instead, the most succinct guide I could find is very involved. I think I need a token authorization/authentication setup because there may be consumers (like apps) who don't have a typical cookie pattern these days. In my head, the token works like a cookie except it's manually held on the user end and passed via header with each request?
To its credit, the guide worked, at least for logging in and correctly utilizing the simple Authorize attribute in controllers. However, User.Identity.Name is always empty, even when User.Identity.IsAuthenticated is true, which is perplexing.
How I think auth is working:
User request hits API with username/password
Service checks the combination, and returns an encrypted JWT to the user
The user sends the JWT back with every request
The server decrypts this JWT to identify the user - this is probably where I'm wrong
So here is where my question comes in:
I need more data about the user, like access to the entire UserModel with every request, but I don't want to go to the database to find it every time. This is where I think there should just be a session object in memory, but that doesn't appear to be the case with token authentication.
TL;DR:
Where do I put user-specific, short-term ("session") information for consumption in future requests where a user is identified with a JWT in the Authorization header instead of a cookie?
Session state isn't right, because it's hard-wired to a cookie
HttpContext.Items aren't right, because it's just for the one request
Cache storage isn't right, because it's not user/session specific. I could potentially create a session-like user-keyed storage here but that seems way, way over-engineered for this.
Basically anything where I'm passing all the data (not just a user identifier) to the client then relying on the client to pass it back seems wrong? But feel free to correct me.

The server decrypts this JWT to identify the user This is probably
where I'm wrong
The JWT token is not encrypted, its signed so you can't alter it. You can open it if you look at jwt.io for example.
Where do I put user-specific, short-term ("session") information for
consumption in future requests where a user is identified with a JWT
in the Authorization header instead of a cookie?
You put it in the principle claims of the token. In the guide you linked it wrote:
var claims = new List<Claim>
{
new Claim(JwtRegisteredClaimNames.NameId, user.UserName)
};
So you add whatever you want to the claims to store it on the token and later you can access this data via:
var claim = _contextAccessor.HttpContext.User?.Claims.FirstOrDefault(d =>
d.Type == ClaimTypes.NameIdentifier);
You also can't use any of these other examples that you listed like HttpContext.Items because those are not signed. If the token is altered in any way the system identifies this and returns a 401

Not getting refresh token from Salesforce

I have been using Force.com Toolkit for .NET for long time. Recently one of a client has started complaining for session invalid issue. So I started digging up and found that I have to refresh token by calling TokenRefreshAsync for which I need to pass on refresh token which I get during authentication. But I am getting null refresh token from SF.
I have tried everything possible thing I found on the internet without any success. Perform requests on your behalf at any time (refresh_token, offline_access) is added in OAuth Scopes:
Refresh token expiry is set at 2 days:
This is the simple code I am using to authenticate:
var task = authClient.UsernamePasswordAsync(consumerKey, consumerSecret, username, password, callback);
task.Wait();
What am I missing here?

The Username-Password Oauth Flow does not provide a refresh token on Salesforce, regardless of scopes:
This OAuth authentication flow passes the user’s credentials back and forth. Use this authentication flow only when necessary. No refresh token is issued.
If you want a refresh token, you'll need to implement a different OAuth flow (preferable!), or eschew the refresh token and reauthenticate when your access token expires. The latter makes you vulnerable to credential and security token changes on the part of the authenticated user, however, which using a more suitable OAuth flow grants resilience against.

How access token is validated for accessing protected resources in token based mechanism?

I want to do token based mechanism where I would be having either SPA or mobile apps supporting multiple clients.
Use case of my web service engine and my application:
My web application: Client will do registration of their application either SPA or mobile apps.They will get client id on registration.Only client id as secret key would be compromised in case of SPA or mobile apps hence I am just providing clientid.
Web service engine: Support multiple client with managing session of each user after login in to respective application of clients.
So let's say there are 2 client who have register their application in to my web application :
Client 1 : MyApp1
Client 2 : MyApp2
Now if MyApp1 have 2 users with John and Stephen and if they login in MyApp1 then i want to manage session for those users with token based mechanism. Now if John and Stephen wants to access protected resource then they can access only through valid accesstoken.
Same goes for MyApp2.
For token based mechanism I have seen lots of question referring to this below article only:
http://bitoftech.net/2014/06/01/token-based-authentication-asp-net-web-api-2-owin-asp-net-identity/
But the only confusion part in above tutorial and in most of the tutorial is after validating user name and password and generating access token. Does above tutorial is storing access token in server side cookie for validating accesstoken when request comes to access protected resource?
I am really confused here. I know accesstoken validation happens inside [Authorize attribute] but I am not getting without storing accesstoken how above tutorial is validating accesstoken.
My thought is like may be when request comes for accessing protected resources access token is encrypted or decrypted based on machine key attribute in webconfig and this is how access token is validated inside [Authorize] attribute but I am just not sure about this.

You can control what information goes inside a token. Look at the SimpleAuthorizationServerProvider class in the article:
var identity = new ClaimsIdentity(context.Options.AuthenticationType);
identity.AddClaim(new Claim("sub", context.UserName));
identity.AddClaim(new Claim("role", "user"));
Use the Claims to store anything you need regarding to the user, their username or roles and this is what happens in the article you referred to.
The token generated already contains that information about the user.
This is taken from the article :
The second method “GrantResourceOwnerCredentials” is responsible to
validate the username and password sent to the authorization server’s
token endpoint, so we’ll use the “AuthRepository” class we created
earlier and call the method “FindUser” to check if the username and
password are valid.
If the credentials are valid we’ll create “ClaimsIdentity” class and
pass the authentication type to it, in our case “bearer token”, then
we’ll add two claims (“sub”,”role”) and those will be included in the
signed token. You can add different claims here but the token size
will increase for sure.
This is why you do not need to store the token anywhere,the token is self contained and everything is stored inside it in an encrypted form. Don't forget that before you add a claim containing the username you have already validated the username and password, so you can guarantee that the token is created correctly for a valid user / password combination. Of course you do not want to store the password inside the token, the whole point of tokens is to avoid doing that. Passing passwords to an API all the time does increase the risk of them being stolen, tokens are much better for this.
Finally, the tokens expire after a time you control, usually they are short lived so even if someone does get their hands on one they will not last long.
If you take care of how you pass the tokens, meaning in the Authorisation Header over an https call then you are as protected as you can be and the headers will be encrypted. The point here is to never issue calls like this over basic http.
The author of the article you referenced is a well respected authority in this particular area and currently a Microsoft MVP and you are basically in good hands. Keep reading his articles, but pay attention to the details.
----------- Clarification related to JWT format --------------
yes the JWT token will contain information related to its issue date and expiry date as well. I have an article of my own on this : https://eidand.com/2015/03/28/authorization-system-with-owin-web-api-json-web-tokens/
Look at the calls which create the token and look at the information returned in the screenshots.
In my example the token contains the actual encrypted token, the token type, seconds it expires in, the audience which is the ClientID, when it was issued and when it expires.
This is just an example of a token, yours will look probably a bit differently but you get the idea I hope. Use Postman to see what's coming back in the token
There are a number of concepts to be understood when it comes to OAuth2, it does require a bit of research and practice.
In short, you request a token with A Basic Authorisation Header, you get the token back and it's telling you what type it is, in my case it's Bearer so that's my next Authorisation Header for any call to a protected resource.
My suggestion is to start small, one step at a time, use Postman to build your calls and understand what's going on. Once you have that knowledge it's much easier to progress. Took me about 6 weeks to wrap my head around all concepts and get something working first time around, but now it takes a couple hours at most. Good luck

The application does not need to store the access token server side, it will only read the user from the token which is passed along.
When the request hits the authentication server, which is attach to the Owin pipeline in the ConfigureOAuth() method,
the HTTP header token is decrypted and the user data from the token is sat to the current user of the context.

This is one of the things that bugged me for a long time
I'm not sure I understand why did you give an example for 2 applications, but the token mechanism is actually simple, but it's kinda black boxed when you use owin and identity
the token is not stored anywhere on the server or the database, authenticating the user on login is done using your logic or usually again black boxed in identity, this involves validating a secured password etc
after this the token is generated (usually using identity) or if you did it manually this will involve securing the token with whatever info you want to store in it
when the user sends a request next time he should pass the token and you will need to decrypt it and validate what's necessary (like expiration time for example), all of this is done behind the scene usually
just a fun note: even if you changed the DB completely the token will still be valid with the user id that doesn't even exist in your new DB! but of course identity automatically invalidates this token when it compares with the securityStamp

Sign in to OneDrive without typing user informations

I need help to obtain an authorize code, is there any way to get one without accepting anything or any window that pops up. I need this for my service as an automated process.
I tried like a thousand ways but nothing works.
Please does anyone know a solution?

As per the docs (http://msdn.microsoft.com/en-us/library/dn659750.aspx), you can use a refresh token if your app has offline access in its scope.
If offline access is set, when you retrieve your authentication token normally, you'll also get a refresh token. When the normal token expires, you can request a new one similarly to the first token, replacing the code query parameter with the refresh_token parameter.
To be clear, the process looks like this as per the docs:
Send your user to your web service
Direct them to the OneDrive authorise page with the correct parameters (make sure offline access is in the scope)
Wait for them to be redirected back to your app with the authorisation code
Exchange the authorisation code with OneDrive (using the oauth20_token.srf endpoint) to receive a set of tokens (one of these will be refresh)
Wait for the access_token you received to expire
Exchange the refresh_token you received for a new access token as per the "Getting a new access token or refresh token" section of the docs

Generate access token without User Interaction for publish-actions

I want to post an update to my own facebook page from a .NET service. I can make a FB App and that wants 'publish-actions' and 'manage_pages'. But how can I get an accessToken in code. The posts would be infrequent so I don't mind generating a new access token each time, but I need it done by the service without any interaction from me.

Following scenario 5 of https://developers.facebook.com/roadmap/offline-access-removal/ should allow you to authorize once as a user and then use the page token indefinitely without any need to re-authorize.
Exchange the short-lived user access token for a long-lived access token using the endpoint and steps explained earlier. By using a long-lived user access token, querying the [User ID]/accounts endpoint will now provide page access tokens that do not expire for pages that a user manages. This will also apply when querying with a non-expiring user access token obtained through the deprecated offline_access permission.
https://developers.facebook.com/roadmap/offline-access-removal/