I think messages from other iphones show up as green, with messages from android phones showing up as blue. No clue how this interacts with group chats.
Literally everything. iMessage is a real messaging app with all of the bells and whistles. Blue means all of the features you want will work. Green means SMS, which is garbage.
iMessage gets you end to end encryption, read receipts, reactions, larger media sharing, functal group chats, and more.
RCS solves some of the shortcomings of SMS but it’s not a feature that Apple supports.
Yes, that's what the whole thing is about. I think most people don't understand or don't want to care about the issue so they just want to fix the symptoms without fixing the issue