The first version of MacOS X Server was based on an unreleased version of NEXTSTEP which in turn used 4.4BSD and Mach 2.5. Around BBB 1997-1998 a lot of userland was synced with bits from not just FreeBSD but the other BSD distributions, if my memory serves me correctly. MacOS X moved to Mach 3. That’s a very very long time ago though, and Apple obviously did a _lot_ of their own CoreOS engineering, things like launchd and XPC don’t have FreeBSD equivalents.

But hey, Darwin is open source so if someone wants to do go on a provenance archeological dig, it could be done!