Who gets the fee
Those who process and record transactions on the blockchain charge a fee for doing it. On Ethereum and similar networks, it's called gas. This fee goes to the network, not to the wallet app.
What the amount depends on
The fee changes with network activity: the more people using it, the more expensive it gets. It also varies a lot from one network to another: Ethereum tends to be pricier; Polygon and BNB Chain are much cheaper.
The catch: the fee is paid in the network's coin
To send USDT on Polygon, you need a little POL (the network's coin) to pay the fee. On Ethereum, it's ETH; on BNB Chain, BNB. Without that coin, the transfer won't go through, even if you have a USDT balance.
A good wallet shows you the fee before you confirm and warns you when you're missing the network's coin.