How are payment systems arranged?

Question

Payment systems

I would like to know how payment systems are arranged in their simplest form: how data is stored, how transactions are made, how replenishment from terminals (or other methods) is implemented. I assure you, I'm not going to create my own system, but I want to learn something useful for other projects.

Answer 1

As well as non-payment.
The data is stored in the database like everyone else.
Transactions in the database.
Contracts with operators and work through api.