> FreeBSD code is also in iOS and macOS via Darwin […]

It is a common belief that Darwin has allegedly descended from FreeBSD, but there is not a lot in there: a pretty ancient snapshot of the FreeBSD userland, another snapshot of the TCP/IP stack that has now completely diverged from the current FreeBSD TCP/IP stack (or, more correctly, the other way round), plus a few borrowed kernel level API's (kqueue is the most notable one).

VMM, VFS, driver layers, file systems etc etc do not share the same lineage.