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.
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 long does a USDT transfer take? On TRON it usually lands within minutes, while on Ethereum the time follows congestion and the fee paid; no network keeps a fixed schedule. Part of every wait is confirmations. Blockchain confirmations are the count of blocks from the one holding a transaction up to the newest: the block that includes it is the first, and each later block adds one. More blocks make a reversal harder, so the receiving platform waits for its own number, often between one and six for bitcoin, before crediting the balance.