A lot of "alternative sources" are BluRay or even 4K Bluray rips. Of course there oftentimes also are Prime Video or Netflix rips but that just depends on availability. Usually there’s a choice. You can also download rips that are more compressed than Netflix would ever be but if you’re out for quality you just don’t download those unless there’s nothing better available.
The first one or two seasons of Better Caul Saul were available as AV1 and the quality was great. It looked basically uncrompressed. Then the next season was a rip from Netflix... The drop in quality was so huge, it's insane.
I mean, Netflix isn’t even doing great compared to other streaming services… their 4K compression is so atrocious, watching on a 720p projector is a better experience than a 4K TV…