I have heard of Darwin, and went back to read up on it to refresh my memory. While it is considered open source, it is also useless unless it is used for Apple's closed source operating systems, as can be appreciated in this explanation:

In the beginning, Apple used to make Darwin available as a separate OS, including compiled binaries, installers, ISOs, etc. that you could install on Apple hardware. However, for many years now, Apple only provides a source code dump, every time a new release of macOS comes out. It isn't even possible to compile this source code, because it depends on Apple's internal build tools and build pipeline. There have been some projects trying to patch Darwin to compile it with publicly available tools, but those projects have all died from lack of interest.

Open Source should be compilable and able to be used, at least that's my perspective, and I just may be wrong.

Here's the article this came from on StackExchange:

https://apple.stackexchange.com/questions/401832/why-is-macos-often-referred-to-as-darwin

Yeah, but that's just the kernel. Anything above that (window manager, the utilities that they didn't outright copy from BSD, apps, ...) is basically closed source.