Are cactus plants used for making buildings?

Yes, cactus plants are used for making buildings. For example, the giant saguaro cactus [Carnegiea gigantea] has a lot of body parts to offer from its mature height of 40 feet/12 meters. Indeed, the cactus is the source for the renowned roof of the historic Catholic Mission San Xavier del Bac of the Santa Cruz valley near Tucson, Arizona.