Maya

When working with a large number of models/polygons, you can replace them with standins. You can create these from the Arnold tab, save one, and load it back in as a standin.

When you want to spread different objects over a surface, you should use Mash, which you can access via the fx menu. If some of these objects are floating above the terrain, you should add more subdivisions to your mesh. one way of doing this is under the smooth Mesh tab and in the subdivision Levels menu. If you only want these objects to be placed in the regions where you want it, you can make a map, you should add a visibility node. You put your black and white map in the Strength map, and in the map helper, you drag the mesh where you distribute these objects over. Once that is done, you set the Map Projection Axis to UV.