Youtube and Youtube TV are not the same thing. However, both support HDR but not dynamic range switching so you will not get HDR if set to 4K with match dynamic range set to ON.
Not exactly accurate, as far as the older ATV 4K 1st gen goes anyway, and I’m talking strictly about regular YT, not YTTV. On my ATV 4K 1st gen some 4K programming is available, but no HDR programming. This is according to the so-called Nerd Stats menu feature. For example:
I can set my ATV 4K 1st gen to Format = 4K HDR with Match Content = none. Result is that some 4K HDR videos show in actual 4K while others don’t, but none show in HDR.
I can set my ATV 4K 1st gen to Format = 4K SDR with Match Content = Range & Frame Rate. Result is that some 4K HDR videos show in actual 4K while others don’t, but none show in HDR.
That is the same result for both scenarios. If I view the same set of 4K HDR videos in YT on my Fire TV Cube 2nd gen or via Android TV built into my Sony Bravia then all of them play in 4K HDR. So the bottom line is that some 4K videos in YT will play in actual 4K (but in SDR) on my ATV 4K 1st gen while others will only play in HD (usually 1440p). There seems to be no rhyme or reason why some play in 4K and others don’t on my ATV. I suspect it could be (and probably is) related to the ongoing feud and corporate competition between Apple and Google.
i don’t have a ATV 4K 2nd gen to test out, so no clue what it is capable of. I would like to see someone with a ATV 4K 2nd gen do this same kind of test to see their results. Apple has made the settings rather complicated and provides very little explanation for the various setting options. Everyone seems to interpret the settings in a unique way, and this only complicates things more. The Fire TVs and Android TVs seem to have better addressed the settings complexity and just work. I’m not sure why Apple can’t seem to do the same, but they tend to make things way more complex than they need to, and every app developer seems to do their own thing with those settings.