My observations from when I daily drove iOS (no more) mirror yours: the incredible amount of cruft that would accumulate was astonishing. At one point I had a device that was majority full of system storage and other data. The same was true across family devices, too.

Some years ago I stopped depending on Apple's purchased downloaded movies for long flights, after an instance of having the files downloaded to the device beforehand, but Apple deciding I didn't have the DRM keys to play said files during a long transoceanic flight. I then moved to storing DRM-free movies in VLC, but iOS prioritized keeping system storage and other data cruft around, and wiped VLC's stored files. Talk about paying for an expensive device and media you don't really own.

I'd imagine the metadata picture that could be synthesized from that data could be extensive in some cases. This stuff is hard and I'm sure there are good reasons for caching things, especially on a device positioned to primarily act as a readily available front end for online stores, but I have a hard time believing that Apple's executing it well.