Can someone explain this thing? How is it different from Prime, and how can I see what Fullscreen adds to the selections of Prime so I can tell if it's worth it. It seems almost as if Amazon is trying to hide this information.
You are misreading the relationship between Fullscreen and Amazon. Fullscreen is an SVOD service owned by AT&T and is not an Amazon product except that it is available through their Amazon Channels platform. Other things available through Amazon Channels are things like HBO, Showtime, Seeso, ConTV, Acorn TV, Starz, etc. Amazon Channels is just a platform to subscribe to streaming services and access them through the Amazon streaming service available to Prime members.