Ask HN: Make your own payment processing gateway

Out of curiosity, I would like to write my own payment processing gateway, like stripe or braintree.

I would like to write a simple MVP just to see what it entails and understand more about the entire thing from top to bottom.

I haven't been able to find any information on this so far. I know there is the PCI-DSS standard, which is not technical anyway, but that's about it. There is no info on connecting with visa/mastercard/... and literally anything else technical needed to run such a project.

So does anybody know where I could find something? Again, I mean technical information that a programmer can take and write code that could run payment processing. I am not interested in licensing information and anecdotes.