Your USDT is on Ethereum — the oldest USDT contract, the one DeFi protocols and many exchanges default to — and you want bitcoin instead: for a payment, for long-term holding, or because you are done paying Ethereum gas on a dollar token. Leaving Ethereum for Bitcoin in one step saves a trip through an exchange.
The two networks share nothing. An ERC-20 transfer cannot reach a Bitcoin address, and no Bitcoin wallet understands a token contract. The route receives your USDT at a one-time 0x… address, waits for 15 Ethereum confirmations and sends bitcoin to the address you gave. The BTC amount is fixed at the fifteenth confirmation, about 4–10 minutes after you send.