Underground biomes takes care of modifying the terrain generation, adding new materials. We will find these new materials, both on the surface and underground, and this will allow us to make many new building and decoration blocks, i.e. stairs, slabs, blocks, buttons and bricks.
With this new generation of terrain, everything will continue as usual, only now we will have new materials beyond stone, andesite, etc. For example, with this mod, more than twenty-four new stone variants are added.