[parkcedar]

Yes, but since it was initially created I believe a lot of it has been rewritten. Eg, the filesystem. I suspect the memory system is different these days too, since macOS handles compressed memory quite differently (though not sure how that gets implemented under the hood).

FreeBSD is like a great grandparent, related but still very different.

[aeadio]

Every extant Unix has been rewritten since the original AT&T code, Ship of Theseus style. We still consider them members of the Unix family, because they can trace their lineage directly. One could built a Git repo showing every code change from the original Unix release through modern day BSDs, if only we had granular commit info going back that far.

In fact, it's been partially done for FreeBSD, https://github.com/dspinellis/unix-history-repo

We could in principle do something similar for Darwin (if we had enough of the historical code), which is the core of MacOS, which is based on NeXT, which was based on BSD with a new kernel. That makes MacOS every bit as much a member of the Unix/BSD family as FreeBSD is.