100% trust based.

A stable coin is basically a loan

Bitifenex says "give me 1 usd and I will give you 1 usdt. For each usdt I will pay 1 usd back at anytime"

And a lot of people trust that they will pay back, especially exchanges.

Same with gold or anything else that is "stable"

People wouldn't trust if they know everything about bitfinex or the other companies, imo