Re: 5 of those with one BFR
“The could have rolled out 5 saturn 5 rockets, and used those to launch the entire thing....”
Better still they could just roll out 5 of the Saturn 5s, send them up, dock them altogether and make a huge giant Skylab!
*for you young ones, the Saturn 5 was so powerful it was able to reach orbit without needing fully fuelling. So they converted part of the rocket unused fuel tank into a space station, that effectively launched itself into orbit becoming Skylab. Suggest you google the videos of it. It was so big they could fly around the inside with jet packs!