What’s the prevailing theory here? I’m going with clones. But for what purpose? Clone everyone and have the “bad” clones live in a sewer sub society? For what purpose? I honestly don’t get it. I need to watch this a couple more times. All the religious references to spirituality only confused me more.
shareRed (or Adelaide) explains the tethered at the end of the film during the showdown: "It was humans that built this place. I believe they figured out how to make a copy of the body but not the soul. The soul remains one shared by two. They created the Tethered so they could use them to control the ones above like puppets. But they failed and they abandoned the Tethered."
As far as I know, that's the only explanation given which isn't much. But Peele's using the tethered as metaphor for commentary so what they mean in the plot isn't all that important.
You can find what Peele is up to in "Us" reviews and articles (a few think he's a bit muddled though there's consensus about his general message) so I won't spoil things here.
Btw, I got the above quote from a subtitle file. Very handy if you want to review something without watching things again.