any desktop version ?
Not yet, but they are planning to release it.
Meanwhile you can use other alternatives like
Blockstream Green wallet and
Specter wallett.
Green wallet recently added coin control for Liquid assets, and you can also use it with jade hardware wallet or ledger nano s.
1)If I do a swap L-BTC to L-USDT, what kind of liquidity is there? How much can i change so that the exchange rate does not change?
2)What is the exchange rate? How close is it to Binance for example?
3)Where can I withdraw the L-USDT? how to change them to USDT erc-20 ?
Go and look for yourself.
I think they are using decentralized SideSwap and Boltz, but you can download app and check there, it's free.
https://sideswap.io/https://boltz.exchange/You can send to erc20 shitcoin address but I would never do that because of high fees.