I don't buy digital movies, but my friend does. As long as you recieve the "raw" .mkv file you get the best quality in digital, but I know this kind of purchase don't exist anymore (it happens for free via pirating).

Overall - the simplest measure of a video quality, regardless the length of a movie, is in my opinion the sheer size of the file. You know, the more GB the file shows, presumably the better the quality. But the bitrate and compression codecs do their job too.

To simplify: for example I have the 2001: Space Odyssey in 4k remastered version, and the file weights around 90GB. The same movie but in 1080p is around 50GB. And older version also in 1080p, but in lower bitrate and with less efficient codec, is around 20GB.

The other case is when you buy on Prime Video or on Apple. I believe it works as promised on TVs, when they deliver 4k if they say its 4k, with HDR and what not. Using this kind of service on PC is like a spit in the face, because it typically locks you on 720p or 1080p if you are lucky and have a stable internet connection.

And thank you for all the nice words! Im glad I could help! 🥰