Mempool

Every node maintains a memory pool of valid transactions that have been broadcast but not yet mined. Miners sort it by fee rate (satoshis per virtual byte on Bitcoin) and pack the most profitable set into each candidate block — fee maximisation is pure profit optimisation.

A congested mempool means higher fees and richer blocks for miners; an empty one means blocks earn little beyond the subsidy. As subsidies decline through halvings, mempool dynamics will increasingly determine mining revenue.