Thanks for your reply.
Why do you think that some brokers does not have BTC/USD? For example, Binance. They have BTC/USDT and BTC/USDC but not BTC/USD.
Take USDT to be seen as a shadow for USD, it works like USD as it is pegged to it and will always cost equivalent to USD. Some brokers do not take the fiat payments directly and choose USDT over USD to keep themselves out of the fiat based regulations they might face if they allow fiat deposits whereas USDT is a token with 1:1 ratio to USD (fiat).