Are all 4K/UHD Blu-rays in HDR?
No, although most newer ones are. You'll want to look at the box. First, make sure you're looking at a newer 4K/UHD Blu-ray and not an older one. It's pretty obvious, since usually 4K/UHD is printed right at the top.
Finding out if the disc supports HDR isn't much harder. In most cases, there will be some kind of "HDR" sticker on the front of the box. This lets you know that the version on the disc is set up to play in HDR if your TV is compatible. Be mindful that many of these movies come with digital versions that you can stream, and that version may or may not also be available in HDR.
https://eu.usatoday.com/story/tech/...you-need-watch-hdr-movies-and-shows/88979112/