Who uses Unix over Linux today and why?

Oxide (https://oxide.computer) uses Illumos, which is based on OpenSolaris.

Why? I guess easier vertical integration and familiarity.

Oxide builds their own hardware and firmware, some of the founders and employees worked for Sun for a long time and developed important Solaris contributions (e.g. Bryan Cantrill and Adam Leventhal, two of the developers of DTrace).

At my last job, I worked with enterprise security software mostly used by large corporations. We had to support a sizeable number of platforms, mostly Unices. AIX was used by a large number of our customers. Solaris had a solid representation, as did HP-UX.

The primary reasons that these companies used these Unix varieties was that they already had heavy iron that used these things, and they work well so there is no advantage to throwing that investment away to move to something else. The other large, and definitely related, reason is that they had existing internal software written for these platforms that is more cost-effective to continue to use than to replace.

Mac users? Which specific Unix™ are we talking about here?

DHL Express worldwide was a huge enterprise user of HP-UX some years ago, and probably still uses it today, because their investment would be too large to write off, as JohnFen said in a sibling comment about another company.

They had many of their business-critical custom enterprise apps running on HP-UX on many HP PA-RISC servers worldwide, at various locations.

Anyone who uses TrueNAS is using FreeBSD. I think it's because of the ZFS license, but there is a Linux variant now, so something might have changed. I guess jails are also pretty nice; I don't think Linux had an equivalent feature until the dawn of modern containerization.