I find that strange to hear because 90% of the sources you find on the internet say that MacOS is FreeBSD based including the FreeBSD wiki.
You may have heard that Macintosh OSX is just Linux with a prettier interface. That's not actually true. But OSX is built in part on an open source Unix derivative called FreeBSD. And until recently, FreeBSD's co-founder Jordan Hubbard served as director of Unix technology at Apple. Now he hopes...
www.wired.com
Even on the apple forums.
discussions.apple.com
discussions.apple.com
Too bad that Apple doesn't have it officially documented somewhere since that would get rid of all the confusion.
OK there seems to be some mix up so, I'm gonna go through a brief history. In order to help clarify things and maybe add why I prefer Linux over MacOS. But BSD is Unix. See what happened is way back during the beginning of Unix System V which is what the majority of the world has used for forever. AT&T would push Unix into the universities along with the source code under the premise that grad students could learn more about OS developement. In reality AT&T benefitted a lot from their code. Eventually they even started making their own unlicensed equivelants of Unix tools (this by the way is where people like Stallman would come out of) and right lesser restrictive licenses. AT&T heard this and of course instantly took them to court and said whatever was developed under Unix even for free, was AT&Ts property... The students and faculty not only disagreed they fired back stating AT&T had no rights.
Essentially BSD(Net/2) is UNİX (Net/1) if you removed all the AT&T intellectual property. USL who has changed hands from AT&T, Novell (of Netware fame), Caldera. Under Novell it tried to claim that BSD was infringing on its intellectual property, a trial was had and it loss. Not that long afterwards Caldera tried to use it to do the same thing again to GNU/Linux... And again the suit was thwarted. Because of that though the Linux kernel was completely rewritten to not only not be Unix... But to loose any resemblence to Unix coding conventions. So while it may share some similar user functionality, under the hood it's not even close. Linix BTW is based off Linus Torvalds love of the Minix operating system which is a POSIX. That is an operating system that's seemingly like a Unix (aka Unix-Like) as it runs the same or similar commands but isn't Unix under the hood. The other side of this is the GNU toolsset which accompanies Linux and comes from the HURD project. Hurd is where you get in to Stallman and others who weren't completely happy with BSD and considered the idea selling out. They wanted to rewrite everything and break away.
So BSD is very much Unix on some level. In fact the lawsuit in 92 has to do with the fact that Berkeley Software Design inç (the seller of the commercial version of BSD known as BSDi) was selling copies of BSD/386 then through 1-800-It's-Unix line. Speaking of adding insult to injury, not only did they have the right to publish this they were gonna tell the world it's genuine Unix. They had to know they were lighting a match with that.
As for GNU/Linux which is a convergence of Linus Torvalds love of Minix (a POSIX OS) with the toolchain / toolsset / utilities for GNU/Hurd (at best a POSIX). POSIX by the way is literally the conjunction of Poser and Unix. So literally it's it's own operating system but that poses as a Unix.. Similar or same CLI. This is why on Unix you have the Bourne shell and on clones you have BASH (Bourne Again Shell, meaning it's Bourne reincarnated on GNU/Systems)...
And a last cavaet. Everything that went into MacOS, and Next Step has paid fees to USL last I heard. That means they licensed at least part of the commercial USL code and they have BSD TCP stacks like 99% of the world. So there's that.
POSIX while feeling the same are never the same under the hood. They're completely different, for example single user instead mulri-user.. POSIX systems are also not licensed because they are not considered UNIX and thus not intellectual property of USL.
As for the last of us being MacOS. Mac at its core uses a lot BSD code, which in turn is heavily based on the System V architecture which dates back to before the 80s. Linux uses a far more modern kernel. I prefer the more modern kernel architecture, but to each their own . The Arch of BSD has been revised but Linux has been completely rewritten twice. So which is better, is any better, and I think that's up to the user. There's features sometimes in Linux that annoy users because they're so modern. And that's fine. Everyone has their level and preferences.
But as for the answer
Linux is a POSIX at best
BSD is at least partly based in generic (non-Ip) Unix code
And MacOS is most probably a UNIX if not a Unixish (meaning not a true Unix but way more than a POSIX)