Fintech understanding

hi , i want to understand how the money transfer apps work and payment getaway so many companies provide payment getaway, i want to understand how they are working. are they integrated with all local banks "witch not logic" or how they provide such services after researching in the local market i found so many payment getaway that any website can integrate and start the payment but this companies take high rate per transaction. what i want to understand how these companies works and how they can provide such services, since i want to provide such service by myself to directly myself. is there any type of integration with local banks or integration with master/credit card i want you to guild me how I understand how this companies works , and how I can start my company for the same business, also i found some application that provide money transfer , how i can achieve that. after that we can think about the system to handle that