How does Paypal/X.com work?

Hi guys,

Can anyone direct me to a resource on how to learn about how Paypal works? or how an online bank works?

I'm a newbie python programmer, finished Al Sweigart's books and one or two others on pandas.

I was reading a book about Elon Musk and how he started X.com (which merged with Paypal and Peter Thiel). I was just curious about the programming structure of how an online bank would work and online payments/ money transfer.

All I remember is learning about RSA encryption for credit cards in University, but don't know much else. I've tried to google but can't find anything about it, so if anyone has any resources, books, videos, articles, blogs, etc on how it's done that would be great.

I'm just curious how they work and learning to build/program something simple would be fun.

Thanks

YS