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.Net Core 3.1 How do I import a .cert and .key file in a X509Certificate2 Object?

This is the first time I am dealing in code with certificates.
My problem is, that I need to sign emails with a certificate that is split in a .cert and in a .key file. Those files need to be read from the file system and cannot be stored in some kind certificate store.
The self signed private key for testing purposes starts like this:
-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-256-CBC,B5F1CE2CAB1B3CE20326EF3CD60D230
tmPJKtI8S4dGl2B29HhyHlF6Dp6/mDldldX/n2+gYvfSaa4TEPVFQMJfLsRxp1ey
...
Importing the .cert part is fairly easy and straight forward:
X509Certificate2 certificate = new X509Certificate2(_emailConfig.PathToCertificate);
But I fail to figure out how to add the private key which I need for the actual signing.
This needs to also work on Linux.
Any help would be appreciated.
Update 0:
I obtained a string called privateKey that only contains the private key without any PEM syntax.
Then I did the following:
var privateKeyBytes = Convert.FromBase64String(privateKey);
using var rsa = RSA.Create();
rsa.ImportRSAPrivateKey(privateKeyBytes, out _);
Then I assigned the key:
certificate.PrivateKey = rsa;
The problem is I am getting the following Exception: System.Security.Cryptography.CryptographicException : ASN1 corrupted data.

How to convert ObjectHandle from Pkcs11Interop to X509 Certificate or Pem format?

How to convert Public key from Pkcs11Interop to X509 certificate or pem format

If you want to extract RSA public key value then you need to read CKA_PUBLIC_EXPONENT and CKA_MODULUS attributes.

Unable to sign data with X509Certificate in python

I do not have much experiance with security, but now I have to implement a signature procedure in python.
I have a certificate somename.cer. I have an c# implementation example of how to sign my string with that string as follows:
CertColl is the collection of certificates where related code finds the related certificate with Thumbprint in the previous lines and returns a list of certificates.
X509Certificate2 cert = certColl[0]
RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)cert.PrivateKey;
return Convert.ToBase64String(rsa.SignData(Encoding.GetEncoding(1251).GetBytes(my_string), new SHA1CryptoServiceProvider()));
my_string is the string to be signed and constructed within the code, but I do not need to add those steps in here
So I am trying to implement this in Python with the help of this previous Q&A
from Crypto.Util.asn1 import DerSequence
from Crypto.PublicKey import RSA
from binascii import a2b_base64
pem = open("some-path/somefile.cer") # I have a certificate with `cer` extension
lines = pem.replace(" ",'').split()
der = a2b_base64(''.join(lines[1:-1]))
cert = DerSequence()
cert.decode(der)
tbsCertificate = DerSequence()
tbsCertificate.decode(cert[0])
subjectPublicKeyInfo = tbsCertificate[6]
rsa_key = RSA.importKey(subjectPublicKeyInfo)
As I expect, now I can sign my_string with this.
rsa_key.sign("Hello World", "")
But I receive the following error:
TypeError: Private key not available in this object
Am I doing something wrong, like usnig the wrong method to mimic rsa.SignData in python?

Your certificate does not contain the private key.
From what I see in your C# code, I'm guessing you're sourcing the certificate from the Windows Certificate Store. This store can contain certificates both with and without private key attached.
.cer files, on the other hand, (usually) don't contain private keys - they only have public keys. That's why signing with it is impossible.
I'm guessing you have exported the .cer file from the Windows Certificate Store and haven't selected the "Export private key" option. You should have better luck by re-exporting it in .pfx or .pvk format and try signing with that file.
See more on this topic here

Encrypting Data with RSA in .NET

I have a DER file with sha1RSA as the Signature Algorithm. I have to encrypt some data using it.
Can anyone tell me how do I load the DER file and use the RSA public key in it to encrypt my data in .NET?

DER or Distinguished Encoding Rules is a method for encoding a data object, such as an X.509 certificate, to be digitally signed or to have its signature verified.
The X.509 certificate only contains the public key. You need the private key to decrypt!
Typically private keys are exchanged in .PFX files, which are password protected.
-- EDIT --
Sorry I misread your question. Yes, you can encrypt with the public key of X.509 certificate. You can load the .der by using System.Security.Cryptography.X509Certificates.X509Certificate2.Import method.
Then convert the public and encrypt, something like:
rsa = (RSACryptoServiceProvider) certificate.PublicKey.Key;
encryptedText = rsa.Encrypt(msg, true);

How can I import private key and use it to sign document in C#?

I've generate a self-signed certificates(X509 certificate) and want to use the private key to sign some document to make digital signature and I'm doing it in C#.
How can I import .pvk file? Do I need to import from key store?
And can I use this code to sign and create a digital signature?
RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
RSAPKCS1SignatureFormatter RSAFormatter = new RSAPKCS1SignatureFormatter(RSA);
RSAFormatter.SetHashAlgorithm("SHA1");
RSAFormatter.SetKey(cert.PrivateKey);
byte[] SignedHash = RSAFormatter.CreateSignature(data);

Try tho Initialize your RSA like this:
RSACryptoServiceProvider RSA = (RSACryptoServiceProvider)cert.PrivateKey;
To generate a signature I use rsa.SignData(data, "sha1");

First of all you need to determine, in what format the signature should be made. If you deal with certificates, most likely you will create PKCS#7 or CMS signature. There's a class in .NET for CMS signatures.
The next steps depend on whether you need certificate-based PKCS#7 signature or RSA (PKCS#1) signature.
Side note - putting several questions into one makes it hard to answer properly. Also most of your questions have lots of answers if you do the search.