tl;dr: Visa and Mastercard operate card networks with four core functions: routing transaction messages between issuers and acquirers via their telecom infrastructure, coordinating banking relationships to settle payments (including across currencies), setting interchange fees and incentives that shape network behavior, and enforcing rules plus arbitrating disputes. Notably, in a typical 2.5% US merchant fee, the issuing bank pockets ~2% while the network itself takes only ~0.15%—which is why issuers fund lavish rewards programs. EU regulation caps interchange at 0.3%, explaining the region's weaker rewards culture and stronger alternative payment adoption.