units on doodads?

im not sure if this is the right place but, in many pictures and some maps ive seen units like peasants and rifle man ect placed on doodads im not sure how to do this so can someone please help me? help is much appriciated

Well, mostly, when we're talking about threads like challenges, and the Weekly Terraining, it's actually doodads, only they have given it the model of a unit.
Otherwise, it is possible to create a destructible and set it to Walkable, and then units will follow it's shape when they walk over it. Although that's mostly used in-game.

im not sure about all the ways this can be done, but for terrain just for screenshots, i always use doodads with no pating then place invisible platoforms inside the doodad and raise them to the appropriate height, i think this is also the way most cinematics use