To calculate the APR, we take the last 30-day increase of the hETH price - which is taken according to (total amount of users' ETH in validators & rewards) divided by the number of hETH minted. The rewards come from the block rewards issued by the Ethereum network for staking ETH combined with additional MEV rewards. These rewards are redistributed to the pool and grant hETH (that represents the amount of ETH + rewards in the pool) its value.