These are a few different model generators built in Houdini. They take in a base shape (or a mix) and convert them to the detailed models.
The poly/tri pieced generator uses some fairly simple re-meshing and individual face extrusions, while the layered shapes are built by converting the base shape into a volume and extracting chunks from that.
Most of the steps in the Houdini generator and the models in Unreal at the end.