They surely hired someone although in most cases the manufacturers aren’t having people actually create models of their products from scratch. Most often thy’re SketchUp files created from some other file type they already have.
They aren’t selling their models through the 3D Warehouse. Consider it more like free advertising.
Due to changes in the file structure with SketchUp 2021, the Warehouse won’t have to convert files made in newer versions to work SU2021. Won’t help users of older versions, though.