Ask HN: Good resources to learn how to build payment based applications

Payment/money based applications have always intrigued me but I am unsure on some of how to implement the "money" part of it. I am clear on using stripe to facilitate getting money but the hard part seems like how do I store and return it.

If a customer provides me x amount of money that they can then use to facilitate real-world purchases through my tool how do I safely store and track that (think PayPal for example)?

I have gone looking and a lot of resources center around the "here's how to implement stripe" which I can figure out. Are there any resources for what comes next? Handling the money, returning, ledgers, etc?