Windows has huge API surface. If you really want to learn it, I suggest reading books, not source code. E.g. “Windows internals” is descent one.

If you’re just interested how particular program works, with the tools that I’ve mentioned in the above comment, you’ll find more about the API than looking at the source code. Especially if you’re just learning. Many apps use big libraries like MFC or QT, the source code will tell you how the developer called these libraries, but the tools will tell you how the process interacts with OS. Due to the complexity of the libraries, can be huge differences between the two.