What earning on crypto is, and where the yield comes from

You hand coins (usually a stablecoin such as USDT) to someone who holds them for you, or to a smart contract, and get yearly interest. The interest does not appear from nowhere: the venue lends the coins on, uses them to secure a network (staking), or pays a promotion to attract new users.

So the higher the rate, the more it is worth asking who pays it, and for how long.