Not directly, no. You would have to add the libraries to a project to see them in Object Explorer.
There are TENS OF THOUSAND of types (classes/structs/enums/other) in the .NET Framework alone. Add ADO.NET, ASP.NET, MVC, Entity Framework, ... and you're probably going to clear 100,000 types.
The best place to browse it all is the MSDN documentation. The top of the documentation tree is
here[
^].