You use the landscape actor. Have you checked the documentation? https://docs.unrealengine.com/en-us/Engine/Landscape/Creation
You can use one landscape for your entire scene or divide it into sections. It is entirely up to you. If the area is really big it might be easier to manage having multiple terrains. Look for “natural” things to hide the seams, like under a big road.
In a nutshell:
- You place a new landscape actor in your scene. There are different resolutions and sizes so you choose one that fits your needs. (You probably only know while you are working on it if it was a good choice so just do a quick test before you go into too much detail.)
- The landscape will be all flat so you can start “sculpting” the height or import a heightmap if you have one.
- You create a special landscape material. It contains multiple materials that are blended with special nodes. There isn’t a clear limit of how many materials you can use. I never used more than 4 myself on one landscape actor.
- Then you start painting your landscape. It all happens in the landscape mode. You select one of the blended materials and paint with the mouse.
- Finally you paint foliage (grass, trees) with the foliage tool where needed. (There is also a different method to paint meshes with the landscape materials but it is a bit experimental with no clear documentation.)