There was more to it than that, but there needn't actually be. It really was a free-standing room, with wall sections that bolted together, so that it would always be the same size, with a double doored light-trap box in the corner for entry. The lens wall had a rail system allowing around plus or minus 15" of lens travel for focus and magnification. Further, the ceiling had a pulley system for hoisting the print through the rollers and up into the air so it could then be laid on the floor right-side up for peeling. The "film holder" actually was a huge metal frame (several hundred pounds assembled and also bolted to the rear wall) that also housed the motorized processor rollers and vacuum board to keep the film flat during exposure.
It really is a marvel, but finicky to set up and run well.