In the main desktop app if you like something via streaming you can download the tracks to storage, but they are in a separate folder to purchased downloads and presumably protected in some way with DRM - if you change the folder accidentally which is easy to do on Mac all those local downloads of non-purchased items are deleted.
It’s just similar functionality to downloading for offline usage in the Qobuz app on a phone.
Even though I have Prime I think they wanted much more, maybe £10+ in the past to stream the hi-res catalogue - may revisit but I got annoyed with the ‘free Prime Music’ selection change a couple of years ago as it messed up everything in the app on my iPhone without warning when I updated not expecting a major change to functionality.
I also pay upfront for a year. I think that saves around 10% on the prime plus music unlimited. Amazon France so I’m not sure what the UK site charges.
They used to flag offers occasionally, but with Apple Music (family) and Qobuz as well as Tidal (now cancelled) I didn’t think Prime Music was worth it. If I didn’t have Apple Music/Qobuz I’m sure it would be attractive except for the fact I’d have no easy way to output the audio to the Nova in hi-res.