Given the volume of material these streamers are handling, I expect QA is minimal. I remember when I was watching Frasier on Amazon Prime, a bunch of the episodes had been configured to play in the wrong aspect ratio. Clearly nobody had ever bothered to check them.
Yeah, I expect QA is minimal for these shows that are past their prime. Only fans will really watch them again, it's probably not worth it to spend the extra time to review every single episode. (But of course, fans will care! I'm just saying it's probably not worthwhile for HBO to check)
I set up custom ingest workflows many cable companies around the world and they all worked the same. You just had to trust that the providers sent you good copies and get them to fix their shit if it was wrong. Most of the time it was bad metadata (episode description, ect).