[Plugin Development] How to build a payment gateway?

Hi all, I’m quite new to web development and I’ve taken an interest in developing plugins. Can someone perhaps give me an insight as to what I need to take in to consideration when I’m creating a plugin with reference to WooCommerce? Specifically a payment gateway. For example, where should I start if I’m making a plugin from scratch, and how should I logically structure making a payment gateway plugin?

I somewhat understand how plugins work, with hooks, and the anatomy of the hook, but I’m a little confused as to how to build one, or rather, read how someone else has built theirs and how their code functions. (I just really want to be able to understand how the code is run, and what kind of functions are needed to make a nicely working payment gateway plugin.)

I realise this is quite a big and broad question, but if I could receive some sort of documentation, and links on how to systematically build one, that would be great!
I’m also looking towards using additional features like having ‘saved cards’ so the users can have an easier time purchasing products on the site.

Insight on security on payment gateways would also be appreciated!