The prices are sort of set at the margins. There was a study a while ago that a relatively minor amount of actual cash can move the btc price (a couple million, despite market cap then in the billions) by carefully driving it up.

I don't know what they're currently printing weekly, but afair they print a billion or so every couple weeks when they're really at it, that's probably enough to substantially move the market, or at least support it.

What happens when there's no more $1B over a few weeks to support BTC when the price is under pressure next time?

I'm not saying that's necessarily the case but the problem with tether and how it's used is potentially much bigger than the market cap indicates.