The Unity Editor includes a built-in set of Terrain features that you can use to add landscapes to your game. You can create multiple Terrain tiles, adjust the height or appearance of your landscape, and add trees or grass to it.
The Landscape system inside of Unreal Engine is a collection of tools that allow you to create expansive outdoor environments, with the Landscape system, you can create terrain for your world. Mountains, valleys, uneven or sloped ground, and even openings for caves are possible.
https://dev.epicgames.com/community/learning/tutorials/M4kp/unreal-engine-tip-13-unity-unreal-switching-terrain-landscapes