Space in a block is limited, and everyone wants into it at once. The fee is the price of the queue: the higher your willingness to pay, the sooner the transfer gets into the network.
Why the amount sent is irrelevant
The network charges for computation, not for money. A transfer of ten dollars and one of ten thousand take up the same space, so they cost the same. Hence the practical conclusion: small transfers on an expensive network are disproportionately bad value.
What you pay with
Always the network's own coin: TRX on TRON, ETH on Ethereum, BNB on BNB Smart Chain. You cannot send USDT without holding the network coin, and that is the most common reason a transfer gets stuck.
Network fee and service fee
These are different things. The first goes to the network and reaches nobody involved in the exchange; the second is the exchange service's charge, and it may be nothing at all, which does not cancel the first.
What it costs in practice
On TRON, usually under a dollar, and with enough network resources staked it can even be free. On BNB Smart Chain, cents. On Ethereum, from one dollar to several tens of dollars depending on the hour.
That spread is exactly what decides the choice of network: on a two hundred dollar transfer the difference between networks turns from a trifle into a noticeable share of the amount.
Who pays the fee in an exchange
The sender, each for their own send. You pay to send crypto to the exchange service, the service pays to send to you. So the network fee is already accounted for in the amount you receive and does not appear as a separate line.
If a service deducts the network fee on top of the quoted rate, that must be stated before the order is created. Such a line appearing at the end is a sign to change exchange service.
Read more: What a transfer really costs on different networks