There are exchanges (such as FTX) where you can trade USDT/USD. The basic premise is that USDT/USD will always be around 1, because Tether (the organisation) stands ready (always) to give you
a) 1 USDT for a USD, and
b) 1 USD for a USDT.
Any USDT/USD deviation from 1 thus gives rise to arbitrage opportunities. Tether can guarantee
a) because they can issue an unlimited amount of USDT, so when you give them 1 USD, they can definitely give you 1 USDT, and b) because
b1) they do not issue USDT without someone giving them the equivalent amount of USD, and
b2) they keep those USD safe in cash, deposits, or equivalent, without price, credit, and FX risk.
a) is undisputed. b) is the tricky part, both
b1) - they could just issue USDT without having received USD. Who's checking?
b2) - they could just take the USD they've received and buy a yacht, or put it in shares (price risk!) or commercial paper (credit risk!) or Chinese Yuan (FX risk!) or anything else. Who's checking?
a) 1 USDT for a USD, and
b) 1 USD for a USDT.
Any USDT/USD deviation from 1 thus gives rise to arbitrage opportunities. Tether can guarantee
a) because they can issue an unlimited amount of USDT, so when you give them 1 USD, they can definitely give you 1 USDT, and b) because
b1) they do not issue USDT without someone giving them the equivalent amount of USD, and
b2) they keep those USD safe in cash, deposits, or equivalent, without price, credit, and FX risk.
a) is undisputed. b) is the tricky part, both
b1) - they could just issue USDT without having received USD. Who's checking?
b2) - they could just take the USD they've received and buy a yacht, or put it in shares (price risk!) or commercial paper (credit risk!) or Chinese Yuan (FX risk!) or anything else. Who's checking?