Rather than being stored in a single file like the accessories, Scorched 3D's landscapes are spread out over a series of different files. Each file contains the different aspects that come together to define the overall result that you see in the game. These aspects include the size/shape of the landscape, the texturing/skin of the landscape, as well as events that occur on that landscape and the objects and sounds that are placed on that landscape.
A brief description and links to more information about these files and what they contain follows:
The landscapes.xml file is contained in the game's (or your mod's) /data/ directory. The rest of the landscape files should be stored in the /data/landscapes/ directory.
In addition to this documentation, a good way to learn to manipulate the landscapes is to have a look at what has already been done. In addition to looking at the standard files that come with the game, there are plenty of mods available to look through as well. Looking at as much of the xml as you can is a great way to get acquainted with the workings of Scorched 3D.