There was a season of This Old House from a couple years ago where they were putting together a kind of pre-fab house. It was somewhat custom, but all the walls and floors and things were made into panels, and assembled in a factory, indoors. They could also set up the wiring and plumbing in the wall panels while at the factory, instead of having to drill into things afterwards in the field. They then just shipped the panels to the site, and basically snapped them together. it was crazy how fast they came together once the panels and stuff were on site.