I should clarify that I am familiar with both modding and programming. However, I'm not sure how one would accomplish what I want without there being dedicated toolkits for achieving things like this.
Say I have a game within which I want to program my own car. The game has no concept of a car, but there are certain things that maybe could be repurposed into a car (Like an animal, for instance, exists within the game and could potentially be modified to act as a vehicle with the right changes) How would I approach this? The game engine is Unity, and it currently has no mod support, but there is an active community that creates and maintains mods as of now.
I just want to know where to start!
@twistedstriker1234's approach is a great option, although I've only tried so far using AssetBundles, which I personally found simple and easy to implement, especially when using the Asset Bundle Browser which can be installed in the editor using the package manager.
However, this approach may have its own problems as for each AssetBundle you need to create a build for each platform (one for Android, another for iOS and so on).
Still I think its a great option as AssetBundles can be virtually anything (audio/video files, 3d models, prefabs, scenes...) which makes it feasible for modding and creating DLCs for a game.