https://wiki.freebsd.org/Myths#FreeBSD_is_Just_macOS_Without...
>Darwin - which consists of the XNU kernel, IOkit (a driver model), and POSIX compatibility via a BSD compatibility layer - makes up part of macOS (as well as iOS, tvOS, and others) includes a few subsystems (such as the VFS, process model, and network implementation) from (older versions of) FreeBSD, but is mostly an independent implementation. The similarities in the userland, however, make it much easier to port macOS code to FreeBSD than any other system - partially because a lot of command-line utilities were imported along with the BSD bits from FreeBSD. For example, both libdispatch (Grand Central Dispatch in Apple's marketing) and libc++ were written for macOS and worked on FreeBSD before any other OS.