Developing a server and the ability to play online for a single player game that doesn't have it?

Question

Game development

Tell me, please, in which direction to study in order to develop a client-server for a single player game where there is no multiplayer?
For example, Euro Truck Simulator 2 was conceived by developers with only a single mode, but third-party developers made a launcher for it ... How to implement this?

Answer 1

First you need to know how client-server games work, how cheats work for the target game. And then work with application memory.
The essence of such client servers is that they write to the memory of the game application, and the game application processes this as regular npc, with specified characteristics, only their behavior is controlled by an external program for writing and reading memory.