Aave/Compound/Curve/Benqi <- decentralized non custodial overcollateralized lending protocols.
These centralized and incorporated companies [basically a big bulls eye saying "please screw me over, here, I'll make it very easy for you for governments"] use Aave/Curve/Compound on behalf of their users who are too lazy/ignorant to do it themselves, plus maybe some economies of scale with cost of gas fee on ethereum.
Right now on benqi (compound fork on avalanche), one can supply DAI and get 8.09% APY in DAI alone (there's more APY in other tokens but I don't like to count those since those incentives come and go), but it fluctuates based on borrower demand/pricing model used in the smart contracts [0].
By supplying DAI, using it as collateral to borrow more dai (up to 75% for dai, different collateral factors for non stablecoins, i think max 60% of collateral value, some with 0%[which means it cannot be used as collateral but it can be lent/borrowed, USDT is like this]) and then depositing it back in again, you can get more yield on that minus the borrowing costs if there are protocol rewards/incentives to be liquidated for more dai.
There's also providing stablecoin <-> stablecoin liquidity on dexes and collecting fees on each swap.
[0]: compounds jump rate model: https://github.com/compound-finance/compound-protocol/blob/m...