This is kinda a broad topic, but I'm kinda uncomfortable about the fact I still don't really understand how all this payments stuff works. Can somebody enlighten me, what does it take, exactly, to build a payment processor? Can anybody do that, theoretically? Do you have to implement some payments API for every bank your customers will use? In every country? I assume, the API's are not entirely open and are more than just a number of SWIFT conventions, so why would the banks work with you, how would you convince them? What really is a payment system, anyway, what's the difference between using a card and making a bank transfer? Why are there so few credit card providers, how are they different from, well, your future payment system?

There are many questions beyond that, but it should be enough to judge the level of my ignorance. So, maybe somebody knows a book, a set of good blogposts, a really long YouTube video? Something?