Season 1: Amazon Prime Video (also, Netflix)
Season 2: Amazon Prime Video Channels
3-5: Prime Video
6-13: Prime Video Channels (with 10-13 also available on the Roku Channel)
14-19: Prime Video (with 17-19 also on Netflix)
20-22: Prime Video Channels (and Hulu, and the Roku Channel)
23-25: Prime Video (and Netflix)
So, they're all on Amazon in some sense. I was aware that there was some kind of concept of Prime Video Channels, but when I tried to find an explanation on Amazon's website, I failed.