This is the best summary I could come up with:

But it hides most of the real Unix directory tree, its /etc is relatively empty, it doesn't have an X server – it's an optional extra.

So taking that list of general characteristics, and adding a less visible one – that it's programmed mainly in C or something C-like – and requiring that the OS looks like Unix and nothing else, meaning there's no other native layer underneath, then the family is bigger.

The original microkernel, CMU Mach, led to a whole bunch of Unix OSes, including the Open Group's OSF/1 and DEC Tru64, as well as MkLinux and famously the GNU HURD.

The only one that isn't a historical curiosity or a tiny neglected niche is Apple's macOS family, including iOS, iPadOS and so on.

QNX is a commercial microkernel Unix-like OS, and it's used in billions of embedded devices … although the only time you might have played with it was Blackberry 10.

A host machine, plus dumb text terminals on serial connections, with no graphics and no networking – even so, high-end kit for the 1970s.

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