there are a lot of BTC deposits in huobi's record, about 944 BTC, but there are only 2 withdraws about 10 BTC
The address has received 944 BTC and has spent all of that.
Note that when you withdraw from an exchange, the withdrawal isn't made from the address you deposited to.
When you deposit fund to your deposit address, exchange sends the fund from your deposit address to their own wallet and when you request a withdrawal, the exchange send you the fund from their own wallet.
does it mean the withdrawal transaction on the website is the selling from BTC to usdt?
Exchanging bitcoin to usdt or exchanging any other coin/token to any other coin/token is done on the exchange, not on the blockchain. So, trades are not recorded in the blockchain at all.