How does crypto lending work, and where does the yield actually come from

Crypto lending works by matching people who want to earn interest on idle tokens with people who want to borrow against collateral they already hold, with a platform setting rates and managing risk in between. You deposit, say, 10,000 USDC into a lending pool. A borrower puts up $15,000 worth of ETH as collateral and takes out your USDC. They pay interest on the loan, and a cut of that interest flows back to you as yield.

Who's actually paying the interest

The interest doesn't come from thin air. It comes from three main sources, and knowing which one you're touching changes how much risk you're taking.

The first and most common source is leverage traders. Someone borrows stablecoins against their ETH not because they need cash but because they want to buy more ETH without selling what they have. If ETH goes up, the trade pays for itself. This is why lending rates on platforms like Aave spike when crypto markets are volatile and traders pile into leveraged positions. More demand to borrow, higher rates for lenders.

The second source is arbitrage and market-neutral strategies. Trading firms borrow one asset to short it against a long position elsewhere, capturing a spread between markets. This activity is less directional but depends on those spreads existing, which means it dries up in quiet markets.

The third source, and the one to watch closely, is other lenders. Some "yield" is actually a new pool of depositors' capital being used to pay old depositors, or a protocol subsidizing rates with its own token to attract deposits. Anchor Protocol's 19.5% yield on UST before its 2022 collapse was largely funded this way: reserves and token emissions, not organic borrower demand. When the subsidy ran out, so did the yield, right before the peg itself failed.

What happens if a borrower doesn't pay back

In DeFi lending, borrowers almost never get the chance to simply not pay. Loans are overcollateralized, meaning you have to post more value than you borrow, and smart contracts liquidate you automatically if your collateral value drops too close to your loan value. On Aave, a typical ETH-backed loan needs collateral around 80-85% of the borrowed value's inverse, and liquidation kicks in around a 1.05-1.1x threshold depending on the asset. The contract sells your collateral, repays the loan, and pays a liquidation bonus to whoever triggered the sale. Lenders get made whole as long as the collateral sale covers the debt, which is why protocols set conservative loan-to-value ratios in the first place.

Centralized lenders like Celsius or BlockFi worked differently, and this is where the 2022 failures happened. They took your deposit, lent it out at their own discretion (sometimes undercollateralized, sometimes to related parties), and promised you a fixed rate regardless of what the underlying loan actually earned. When those loans went bad, there was no smart contract to force a clean liquidation. You were an unsecured creditor waiting in bankruptcy court.

Where does the risk actually sit

Three places. Smart contract risk, since a bug in the lending protocol's code can drain the pool regardless of how healthy the loans inside it are. Collateral risk, since a fast enough crash can outrun liquidations, leaving the pool with bad debt. And counterparty risk, which is really a solvency and honesty question specific to centralized platforms, where you're trusting a company's balance sheet instead of code you can read.

What to check before you deposit anywhere

Look up the actual borrow utilization rate for the asset you're depositing. If utilization is near 100%, the yield is real but you may not be able to withdraw immediately. Check whether the yield includes token incentives on top of organic interest. Read the protocol's liquidation threshold and loan-to-value limits for the collateral types in the pool. And if it's a centralized platform rather than a smart contract, ask what it does with your deposit. "We lend it out" is not an answer. The Aave documentation and on-chain dashboards like DeFiLlama both show utilization and rate history for free, so there's no reason to guess.