Gas measures computational work. A simple transfer costs little; a complex DeFi interaction touching several contracts costs much more, and the total fee is that work multiplied by a price per unit that floats with congestion.
On Ethereum the fee has two parts since 2021: a base fee that is burned, removing ether from supply, and a priority tip that goes to the validator. Heavy network use therefore reduces the total supply of ether.
Fees make small transactions uneconomic at the wrong moment. A twenty-dollar swap during congestion can cost more in gas than the trade is worth, which is the practical reason most retail activity has moved to layer-two networks.