What is the best way to call functions of another contract which is already deployed in the network?

Write an interface for it and create a reference to the contract with the interface

Thanks mate 👍

that doesn't really answer the question, as far as I can tell.
There is no way to make anything "automatic" in the sense that all actions have to be triggered by some external user.

^ this. Smart contracts don't work without direct interaction through a transaction.