Hey Zerbu, a world (Oasis Spring, Willow Creek) is comprised a collection of the .world files in data/shared/worlds/areas. The world map itself is all done via the UI (which is in UI.package). Once you choose a lot/area to play from the world map, then the appropriate .world file is loaded.
If you want to start down the path of modding worlds, I would suggest modifying the .world files in place to see what you can do. Adding of new worlds is tricky because of the UI is somewhat hard-coded to the pre-authored world. Also note that .world files do not work with the Mods directory.
See an old post I made in
this thread for general comments on world modding.
Thanks.
