How are unofficial multiplayers developed?

Question

Programming

How do you write unofficial multiplayers for games like gta 5? Namely, how the game is "hacked", whether the open source code of the game is needed for this, etc.
Is it possible to create an unofficial multiplayer for any game? There are games that do not have official multiplayer (there is no multiplayer game), but unofficial multiplayers for these games are in great demand.

Answer 1

Your questions show your illiteracy at such a level that it will not be possible to adequately explain to you.
In short, unofficial multiplayers are written in different ways. Of course, if you have the source code, things will go much faster, but without it, reverse engineering is used. Studying how the game works and, accordingly, intercepting calls to ordinary functions, while writing your own.
You can create an unofficial multiplayer for any game. But this may require different efforts, and the main question is whether there is someone who is really interested in it, and whether he has enough knowledge.
Again, unofficial multiplayer is poorly monetized due to the impossibility of selling through official channels (after all, the license is often violated), so an ordinary programmer is unlikely to do this, and a good one will most likely be busy with something else.
PS This answer is sufficient and detailed. If you need to understand deeper - learn programming, then you can ask better questions.