Terrains are sophisticated 3D object meshes in Unity. Since the position of terrains is based on a corner, if we want the terrain to be centered on a particular point such as (0, 0, 0), we have to subtract half the width from the X-coordinate and half the length from the Z-coordinate. So, for our 100 x 100 terrain, we set its corner position to (-50, 0, -50) so that its center is (0, 0, 0), which is where our Cube was located.
Textures are painted onto terrains through Terrain Layers. When there is only one Terrain Layer, the texture is used to paint the entire terrain, as it did when we set the first Terrain Layer to the SandAlbedo image. Once there are two or more terrain layers, we can use the brushes to paint different parts of the terrain with different textures.
The "softer" the edge of a brush, the more mixing there is between the texture that was present and the new texture of the brush being applied. The softer outline brushes create soft edges...