This is from the link I provided: “The BSD portion of the OS X kernel is derived primarily from FreeBSD, a version of 4.4BSD that offers advanced networking, performance, security, and compatibility features. BSD variants in general are derived (sometimes indirectly) from 4.4BSD-Lite Release 2 from the Computer Systems Research Group (CSRG) at the University of California at Berkeley.”
After NeXTStep was adopted by Apple, they hired a bunch of the FreeBSD core developers and updated the BSD service and userland using FreeBSD. Apple was actually already messing around with Mach prior to that with MkLinux so there was some initial speculation that they might port to MkLinux rather than update NeXT's Mach+BSD hybrid.
The code updates are very limited, though. To the point that for one's own sanity it's better to assume userland is 4.3BSD unless marked otherwise (been burned by this myself in code that assumed every BSD had changes from ~1994 NetBSD)