I'm not entirely sure which is why I ask how it compares. My understanding is that it's basically what powers MacOS but made up of everything Apple makes open source boxed up together for you as a full system. For the gaps where Apple doesn't release source, homebrew code is written to piece it all together.

Like, PureDarwin is aimed at making Darwin an accessible kernel whereas the OS linked above is more like ReactOS as OP said. I feel there'd be a lot of overlap in work though and would love to hear more about said overlaps