unicornlox Do we have to do it ourselves with code or do we have to use plugins?
Yes. One or the other or both of those. There isn't a built-in 3D terrain implementation in Godot, but there are a lot of solutions out there. If I were you, I would find a tutorial. This video you linked is just a showoff with some high-level how-to. If you are not already familiar with the concepts, you will not be able to understand it. Look for a 3D terrain tutorial that explains everything from the bottom up.