It's literally in USDT contract [1], see line 264. USDT is implemented as a contract in several other networks like ETH and has a blacklist. So every USDT operation is a record in ETH blockchain that calls one of the methods (like "transfer" or "add to blacklist").

[1] https://etherscan.io/address/0xdAC17F958D2ee523a220620699459...