Thank you both, WilliamEdwardsGIS and Kovoschiz, for commenting. :-)

“Showcasing them requires more than the layers currently shown. You need overlays, buttons, and detection. Clickable shapes for per-building view, since level= currently shown for all buildings may not match. Essentially changed entirely.”

– This I think could be done with a layered approach and zoom in/out feature. When a user has enabled a special layer for peeking inside buildings/underground areas, the zoom feature could be used to basically just scroll deeper and shallower. I think most buildings, tunnels etc. are built in a standard way: the base unit is the height of an adult human, plus some extra. A room is typically something like 2-3 meters in height. That is the base unit, so for example a hall at a shopping mall can be 3 x base unit. The zoom feature could simply move deeper/shallower one base unit per zoom event. Clickable areas, detection etc. are redundant imo.