Darwin is the base for macOS and allied Apple OS

Darwin is an open-source Unix-like operating system first released by Apple Inc. in 2000.

It is composed of code derived from NeXTSTEP, BSD, Mach, and other free software projects code, as well as developed by Apple.

macOS is (along with iOS and tvOS) powered by a piece of open source, BSD-based software called Darwin. 

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