I mean the built-in libraries for .NET (whether that is the older .NET Framework, or .NET Core - now labeled just as .NET). One of the biggest benefits to using C# and .NET is the amount of documentation available. If you are still using .NET Framework, and haven't moved to the open-source .NET, I would suggest spending some time learning the open-source version. It's not vastly different, and the good thing is the C# language hasn't changed very much, other than adding new features for developer ergonomics.