I have seen detailing, possibly on HN, a few years back of how tether's asset base was fabricated. The report used intra-day banking movements to show that it was fiscally impossible for tether to be backing their assets at the rate they were minting using the banks they claimed. The report further correlated tether minting to Bitcoin price action. It showed in fairly clear and granular detail how tether was used to pump and subsequently dump Bitcoin and other cryptos. This was back at least 3 years ago. The pump and dump works to allow tether to profit enough to actually get the assets though. And it works until it doesn't. USDT can support Bitcoin price long enough for the minters of tether to extract USD for what they minted as long as their are other buyers getting in.

It is a traditional ponzi scheme but with an additional layer that makes it far more resistant. It's essentially the same thing as government central banking but for crypto and without the threat of violence to control valuations.