What "frozen USDT" means
Frozen USDT usually means Tether has put an address on the blacklist built into the USDT smart contract. The USDT contracts on TRON (TRC-20) and Ethereum (ERC-20) include an addBlackList function that only Tether, as the contract owner, can call. From that moment, every attempt to send USDT from the address fails; the balance is visible but cannot move.
The same contracts also contain destroyBlackFunds, which lets Tether burn the balance of a blacklisted address. That is why a Tether freeze is different from an exchange restricting your account: the first is enforced on-chain by the token issuer, the second is a platform decision about your account.