The macOS userspace was never forked from FreeBSD or any other BSD. If anything, it was forked from NextSTEP. In actual practice, it is a collection of individual components taken from a variety of sources. When development of Mac OS X began in 1999, most command-line tools and a large part of libc were derived from either NetBSD or OpenBSD via NextSTEP. Over the years, there has been a shift toward FreeBSD. Apple maintain a collection of GitHub repositories of their Open Source components where you can see the evolution from one release to the next. Most of them have XML metadata indicating the origin of each individual component.