If a massive standard library, or really any library, doesn't have good documentation how do you find out about it? In some statically typed languages like C#

If a massive standard library, or really any library, doesn't have good documentation how do you find out about it? In some statically typed languages like C# and Java you can use an IDE's object or interface explorer to see what the classes, methods, and properties of a class lib are. If that's missing do you have to use introspection or reflection? In a REPL if the language has one? Is it assumed a compiled language will be statically typed (though that doesn't have to be true) and if it's not the source code will come with any library so examining it is easy?