You are right, although doesn't look like jails was included.

Darwin and MacOS aren't really all that related to NeXTSTEP, on purpose.

NeXTSTEP had proprietary UNIX code in it, and required a UNIX license from AT&T to distribute. Additionally, the display manager (infamously) was based on Display Postscript, which also required a license from Adobe.

My understanding (which is by no means 100% certain! the following is my best guess) is XNU and Darwin were almost complete "rewrites" of NeXTSTEP, preserving the "idea" but with new code.

NeXT's Mach 2 based kernel and BSD userland, which seems to have been very similar to the system developed by Avie Tevanian at CMU and used on their VAXen, was replaced with XNU, which used a Mach 3 derivative from DEC's OSF/1 project, coupled with a new (non-AT&T encumbered!) BSD userland and kernel "module"/personality based on an amalgam of then forks of 4.4BSD-Lite/386BSD, namely FreeBSD and NetBSD (several big bits of libc are from NetBSD).

Point being, I doubt they've copied/pasted large chunks of Free/NetBSD into Darwin/XNU since the late 90s/early 2000s. There was been code flow between the two, but I doubt they'd backport big features like jails (and linux emulation).

(not to mention that Apple's own MACF framework seems to take the place of jails in the code -- see https://github.com/apple/darwin-xnu/blob/main/bsd/kern/kern_... vs https://github.com/freebsd/freebsd-src/blob/main/sys/kern/ke...)

Yeah, it feels like only the minimum needed for Unixy BSD support was adopted from FreeBSD and included in XNU and the userland.

The thing that’s a bit surprising to me is that this didn’t include FreeBSD’s Linux emulation

I don't think it is, they started with NeXTSTEP as a base and just copied interesting bits from FreeBSD. If FreeBSD was the base then that would be surprising.