How to sell USDT for cash usually follows one of two routes. On a regulated exchange, the seller deposits USDT on a network its deposit page lists, sells on the market and withdraws the money to a bank account, after reading the fees and withdrawal limits. Selling to another person through the marketplace of an exchange keeps the coins in escrow until the seller releases them. A seller releases nothing before the money shows in the bank account itself, since screenshots of payments are simple to fake, and payments from third parties can get a bank account frozen.
How to deposit bitcoin to an exchange starts on its deposit page: choose bitcoin and the network, copy the address shown for the account, send from the wallet on that same network and wait for the confirmations the exchange requires. How to withdraw USDT from an exchange needs more care, because a mistake moves the coins out for good. Paste the receiving address, pick the network the receiver actually supports, read the withdrawal fee and confirm with the two factor code. A small first withdrawal to any new address limits the cost of a mistyped detail.
How to convert BNB to USDT depends on where the BNB sits. On an exchange, a convert or trade screen sells BNB for USDT and shows the price and fee before confirming. From a wallet the owner controls, a decentralised exchange performs the swap on chain, with the network fee paid in BNB, and the price filled can differ from the quote through slippage. The USDT contract address should match the one published by official sources, because a name and symbol are easy to copy. Keeping a little BNB back covers later fees.