I just watched this for the second time yesterday and I still also have questions! In Red's monologue, I think she said that she (Red) understood that humans were experimenting and found a way to copy the body but not the soul (hence the tethers) and the idea was that they were going to use those underground to control things above ground, but that the experiment went awry and was abandoned. The tethered, then were left "for generations" and just fended for themselves down there, I guess?
I don't think that everyone had tethers, but they had reproduced enough to apparently do a Hands Across America redo. Red says that she and Adelaide were born special, and they were brought together by God the night of the switch. Do you think young Red was able to control young Adelaide, and that's how she affected the switch in the hall of mirrors? Then she would obviously still control her from underground, but new/adult Red had the soul and was able to lead the others into an uprising, because she would be the only one who knew of the upper world, and of course Hands Across America. (new question, how does she lead an uprising if she's tethered?).
Also, I'm still not sure what actually happened with the dance they talked about? There seemed to have been some big miraculous Godly intervention, but it wasn't clear to me what it was.
Last question (for now) - Did adult Adelaide know she was from the underground the whole movie? I think she did, and it made the second watch more interesting.
Lupita's performance is just absolutely beyond. She's terrifying as Red. This whole thing was gold! But yes, QUESTIONS!