How To Add Terrains To The Editor

Can someone explain to me how to do this?

 

You mean the PC editor? There was a whole livestream about how to use basic route-building features:


Also there's a separate forum for the editor stuff:
https://forums.dovetailgames.com/forums/pc-editor-discussion/

 

but it does not explain how to get some of the terrain files

 

For that, you will have to search the internet and get data for your area of interest. That's still relatively little effort compared to some other tasks needed to model a real world route.

 

This thread, and the unofficial guide it refers to (created by DonnyDave) is very good.
You should be able to get terrain and google map overlays into the editor if you follow the instructions.
Have a look.
Tutorial - (unofficial) Terrain Creation Guide For Beginners | Dovetail Games Forums