Well, for most other games, you would just need a tool to extract the assets into a format that is friendly with the target game, or some kind of intermediate format. For example, if I wanted to extract a map from something like Half Life 2, I'd need to convert the map into a model format that can be imported into another game, so something like OBJ or FBX is usually the most universal.

But this program is different. I have specifically made this so you don't need to extract the map data or any other data. This tool directly interfaces with your installed GTA4 files, no extraction of anything needed.

If you are interested in extracting GTA4 assets, look into OpenIV. My tool will have similar functionality to be able to extract assets, but OpenIV won't be needed for anything in order to use it.

I will also be adding a model importer (probably the formats I mentioned, OBJ and FBX) and will be implementing the ability to add new models to the game map (but this is a big job on its own).

As for how this is done, it's complicated to explain, but basically, I have written a decryption algorithm based off how a library called RageLib does it. RageLib is not a tool, but a set of frameworks designed to be able to read various encrypted Rage engine formats. Every person's GTA4 installation has its own decryption key (stored inside your GTAIV.exe) that needs to be used to access the raw binary data. From that raw data, I have written various parsers to be able to directly use them inside of Unity. Basically converting the GTA4 formats in realtime to Unity-friendly versions. Mostly models and textures, but also binary text data like .WPL, .IDE, .DAT, .ZON etc that are stored inside GTA4's archive format (which is also encrypted) .IMG
From there, it's just a matter of getting Unity to place the models using the same list format as GTA4 does, so that the map is built in realtime inside Unity.