when it first came out, it was only aerial (what's now cityscape and landscape), the others (space and underwater) came later, and I guess they decided not to change the name. even the option to select which kind you want is fairly new, I think there was an entire year (major tvOS version) when you had all 4 kinds with no way to select.
they do offer them in HDR
they play in whatever mode is set in the video settings. If you pause something, they'll even change the rate/range away from whatever you're watching if it doesn't match the default mode. (default in HDR, watching SDR, pause it, Screen saver starts in HDR, when you hit play again, it switches back to SDR for playback)
But... the aTV is really bad about downloading new ones, so if you've got a bunch of SDR downloaded, and switch the mode to HDR, it might only play one or two. There have been many threads here "how to force download new screen savers". and there hasn't really been an answer that works. Possibly doing a full factory reset, and then changing to the mode you want immediately when you start to set it up again.
up until this current OS version, it would pick one that it had downloaded in whatever range you were in. For me, when watching a dolby vision program, it only had one screen saver that it played on loop. not a huge deal because if I've paused the TV, I'm not really watching it. (I have my aTV in SDR, with match rate and range enabled)
