Visa and Mastercard: an approval is a promise, not a payment

How a card network routes about 434 billion transactions a year between merchants' banks and card issuers, roughly 14,500 financial institutions on Visa's network alone, inside a sliver of a 2-second checkout, why no money moves until netted settlement a day or two later, and how tiered timeouts, stand-in and one idempotency key keep a silent bank or a retry from failing or double-charging anyone.