When you look at the page of any variant, you will see a button 'View/Download code' to the lower right of the map. If you click on it, you will get all the files that are used in that variant. The first variants were created by hand.
However, nowadays the site owner (Oli,
userID=5) has made great effort into making variant creation easier. With the help of some tools, you can create variants now without needing to type code once. I have made a guide explaining the current process of variant creation on vDip:
https://docs.google.com/document/d/17oFVvGE8w2HQU-5IeNecFxl4x7VZ0OM2ApjqXY9gi6A/edit?usp=sharing