Where to find Microsoft .NET (C#) BCL reference, what classes are available in BCL packages?

Question

Some classes such as System.HashCode are only available in recent .NET/.NET Standard versions. In the case of HashCode, its is available as of .NET Standard 2.1. However if you include the Microsoft.Bcl.HashCode nuget package, this class can be used in .NET Standard 2.0.

I could not find a complete list of:

1. The BCL packages that are available
2. What classes / features they provide
3. To which .NET target framework (.NET Standard, .NET Framework ...) versions can the BCL package be added

Essentially, the question is: How do I know that a particular class is available for an older .NET via a BCL package? (... and what is the package name?)