macOS runs on the Mach/xnu micro kernel and is pretty successful with it.
Apparently there have been attempts to make a free OS based on Apple’s kernel, but wikipedia mostly talks about them in the past tense. Too bad, it would’ve been good to have such an option.
Pure Darwin ist still around.
I tried out a Darwin distribution a few years ago. It was a BSD with some apple flavor. None of the GUI was included, not all drivers, firmware, etc.
The community is tiny. There was also little incentive to try and fix things or add features, because upstream Apple ignored it pretty much. Grabbing the sources and compiling them into an operating system has little documentation from Apple.
Mac OS X used to install XQuartz, a hardware accelerated Xorg/X11 server by default in the 2000s, but dropped it at some point.
Even back when OpenDarwin and such were around, people would rather install YellowDog Linux that supported PowerPC Macs.
I think at some point the old NeXtStep/OpenStep folks left Apple and the new engineers didn’t understand Unix or think it’s important.