What's the `darwin` in Mac?

In my Mac I execute the code:

import sys


print(sys.platform)

then get darwin, why it is not OSX. I don't know where to look for documentation on python sys module.

What's the darwin ?

"Darwin is an open-source Unix-like operating system first released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, Mach, and other free software projects.

Darwin forms the core set of components upon which macOS (previously OS X and Mac OS X), iOS, watchOS, tvOS, and iPadOS are based. "

https://en.wikipedia.org/wiki/Darwin_(operating_system)