These are ready-to-go assets. You simply drag them into your project, and the terrain, textures, and trees are already set up.
If you prefer using a 3D mesh instead of Unity’s native Terrain component, Sketchfab offers thousands of downloadable "scanned" real-world environments (photogrammetry). Understanding File Formats download terrain for unity
When you download terrain data, it usually comes in one of three forms: These are ready-to-go assets
A standalone generator that allows you to sync your terrain designs directly into the Unity Editor. and the terrain
A highly optimized shader system that makes downloaded textures look significantly more realistic.