You can't, since BTC is on Bitcoin network only, and you can't find it on Tron, Ethereum or any other chain, you may find pegged versions but they are not real and doesn't have security of native BTC.
Cross chain bridges and DEXs have existed for a while. You can swap from USDT on Tron to BTC through Symbiosis.Finance. There isn’t a direct USDT/BTC liquidity pool so it will require several swaps in the background, which will cost you a hefty amount in fees. I would only recommend going the decentralized route if you intend to swap larger amounts, a few hundred dollars at least. For smaller amounts, instant exchanges are a better option.