I bet they might be using scripts. Back in R12 days, that was how we set all our layer states. While not combining all floors in one, for architecture we would combine existing, demo, new, ceilings, finish, power, furniture, etc all into one file. About 100 layers which get sorted out into at least 6 separate printed drawings, 7 if you count presentation. We would use scripts to quickly change layers states and the scripts used wild cards to say turn off partial height walls in the ceiling plan by example: -layer > off > *prht*
the main downside was that it limited the number of people that could work on the drawings at one time. for that reason I now separate most information into separate files and xref for ceiling, furniture, power, etc.
Dan