I don't know what the other user is talking about it being a "separate game." It is heavily data-focused though. It's mostly C# code. As long as that doesn't make calls to Unity functions, it's fine, but also I'm certain they, and other mods, have. However, you can still handle this, similar to how WINE can run Windows applications without running Windows. You have a compatibility layer that routes all the calls that would have gone to Unity to your own systems that behave as close to identically as possible. If you make a call to "UnityEngine.Vector3", for example, you just create your own "UnityEngine.Vector3" that replicates the functionality.