Lots of movies sold on 4k bluray are upscales/‘remasters’ of the 2k version. Some are re-scans from the original 35mm film, those can be pretty good, depending on the source material. There is a huge variety in the quality or 4k movies.
If you can get your hands on it, try the LOTR 4k extended editions. If you get them from an ‘alternative source’ make sure you get one with untouched video (e.g. a remux). They are huge, about 100-120GB per movie but they look amazing. Wonder Woman 1984 also looks really good in 4k HDR, especially the opening scene.
4k content on streaming services varies a lot in quality, but is generally not as good as 4k bluray. Amazon Prime Video looks quite bad, terrible compression with lots of artifacts. Out of the streaming services, Apple TV+ has the best 4k video quality by far.
The LOTR 4k bluray is in my opinion on of the best showcases, especially if you compare it to the HD version. The HD bluray looks good, don’t get me wrong, but that’s all it is. Just a good movie with nice pictures. In 4k HDR with Dolby Atmos it’s something completely different. It’s like magic, almost impossible to look away from the screen. You end up starting it up just to see what the quality looks like and before you know it you unintentionally watched the entire extended edition.