How to automate payments to any paid service?

Question

Python

The question is, of course, more abstract. To begin with, understand the mechanism, and then delve into the details. Let's say there are 100 accounts of different people, every month you need to deposit money for some service.
Where to start to do it manually?
find out if this service has an api for such cases?
what to do with the payment system? In short, a dark forest for me so far.