iPad Pros, in fact all iPads run iOS. Macs, the computers, run Mac OS. Everything else; iPads (all types), iPhones, iPods, even AppleTV run a version of iOS. (iPad OS & TVOS).
All Apple OSes are derivatives of macOS (Mac OS X) at the end of the day.
I'll grant you that iPadOS is still mostly a fork of iOS. But the two are going to diverge more over the course of time. Hell, it wouldn't surprise me if the iPad mini diverged from the rest of iPadOS over time given how many elements from larger iPads just aren't a thing on the iPad mini.
The Studio Display absolutely runs iOS. No question. Same with (at least) the (first generation) HomePod (if not the HomePod mini and second generation HomePod).
tvOS has definitely diverged more substantially. It's true that the pre-tvOS second and third generation models ran iOS. The fourth generation Apple TV (aka Apple TV HD) and all three generations of Apple TV 4K decidedly run tvOS which is different enough from iOS to not be iOS anymore.
embeddedOS, the OS that runs the Touch Bar on 2016 and 2017 Touch Bar MacBook Pros shares its roots with watchOS. bridgeOS, the OS that the T2 Security Chip runs on T2-equipped Intel Macs shares its roots with watchOS mostly (pretty sure there's some iOS in there too, seeing as the T2 is effectively an A10 Fusion).
But, at the end of the day, watchOS, bridgeOS, embeddedOS, tvOS, iOS, and iPadOS are all macOS (Mac OS X).