What is Linux’s native GUI API?

Question

Both Windows (Win32 API) and OS X (Cocoa) have their own APIs to handle windows, events and other OS stuff. I have never really got a clear answer as to what Linux’s equival

Answer 1

To paraphrase the original question:
Q. What is the Linux equivalent of the Microsoft and Apple application programming interfaces?
A. The "Linux Kernel API." https://www.kernel.org/doc/htmldocs/kernel-api