Every flight simulation designer has to deal with the challenges of creating a terrain - ideally something interesting. There are several methods you can use.

One method, used by Microsoft Flight Simulator (MSFS), is to subdivide the landscape into squares. Each square has a geometry and a texture. There are several challenges with this method. One is to insure that the squares fit together without gaps, especially as you get closer. Another is the amount of time required to collect real world data - a task which the MSFS programmers spent decades to complete.

Another method is to create a flat plane and superimpose terrain models on top. This works especially well with certain types of terrain, such as islands in the middle of an ocean. One advantage is that this allows you to create more detailed models of items of interest. Later versions of the MSFS allowed users to superimpose models on the the existing terrain.

Here are discussions of general topics relating to flight simulation design: