How do VPN services (apps) work on mobile devices?

Question

linux

Good afternoon.
As a novice IT Specialist, I am very interested at the moment, how are VPN services arranged?
So you downloaded the application to your smartphone, discovered what life cycle takes place in the server at this stage?
Does the server automatically create a new user? How it works? - Please leave some links, I want to read about it.
Here, the user paid for the VPN connection service for a month, how will the server know that the payment has been made, that on October 28, if there is no payment, they will not let the user in?
I beg you, tell me more about this.
Thank you very much in advance!

Answer 1

They can work through opevpn, they create a user when paying, after the expiration of the user is deleted. This is like an example of work.