question says it all.:yes:
Mods are made when you take an existing game, reverse engineer it to the maximum amount that is allowed, and then build the game back up with new content using the exact same engine or even the same assets, just having a new name, new maps, new missions ect…
Fan games use a custom or existing indie development engine and started from scratch, though it’s possible a lot of models or textures get imported from the game in question, in these cases the engine could even allow features that are not in the original game because it’s using a different engine.
I see. Thanks a lot Ace!