They are still individual grids; they just all happen to be of the same terrain for the day, and adjacent grids with the same terrain type will not show the boundaries between them, but that does not make them a single grid.
If you use a mapping tool that displays S2 Cells at L14 grid size, you’ll see where the grid boundaries are.
The game does not appear to have any kind of algorithm that would cause the terrain chosen for the day to have any kind of preference according to the actual geography of the real world—it’s basically just RNG 1/3 of a chance for any grid to be any of the 3 types of terrain (see https://community.monsterhunternow.com/t/stuck-in-the-desert-biome/28571/6).