[macdice]

I haven't followed it or tried it, but supposedly you can boot up a pure open source Darwin system. Certainly it has been very useful for me as a cross-platform developer to be able to browse its source and understand quirks of system calls etc. Are you saying that key parts you'd like to review are closed source? Hmm, the new file system is closed source, which is a clue, but I don't know to what extent Darwin is "different" from what macOS is really running...

[brirec]

I haven’t kept up with this too much lately, but to my knowledge there is very little documentation or working code to set up a Darwin environment from scratch with FOSS, and there aren’t any modern Darwin “distributions” because of this difficulty and lack of documentation and code.

I wish this weren’t the case, but whatever PureDarwin has right now is probably about the closest you can get to a Darwin environment that boots without nonfree code.