unlike, say, illumos, macos doesn't contain anything genetically descended from the v6 unix kernel; the darwin kernel is mach, with chunks of freebsd grafted into it, chunks that carefully reimplemented without copying the functionality in original unix

by contrast, 'chromeos' and android run on fairly normal linux. they even keep up with linus's tree, and though they do have their own patches, they try to get them merged back into linus's tree