I want my user to send me a smart contract to pay 12000 USDT and get 1 BTC, then I checked his request, which might take long and once I confirmed, I get his 12000 USDT and send 1 BTC to him in a second. Does Stellar support this transaction or XRP or uniswap?
I'm not completely sure what you're trying to achieve. If you have a trustline to a USDT anchor and a BTC anchor, then you can simply make an order to sell 1 BTC for 12000 USDT.
If you want to make sure that either the whole trade happens at once or nothing happens, then you have to build a multisig transaction. The transaction could consist of one operation where the user sends 12000 USDT to you and you send 1 BTC to the user. Now both you and the user sign this transaction, and either both payments succeed, or none is executed.
Stellar itself has no integration with XRP or Uniswap.