Building floors on empty Z tiles?

I was wondering if there is a way to build floors etc on the empty Z tiles such as to be able to create bridges around apartments, build a base on a silo etc?

Good question.
Have you tried the “build” menu? The one called with *
I think you can build more the more skill you have. I m sure I see “bridge” in some menu.

Bridges require water to be below them it seems.
I think there is no buildable floor that can be put at empty tiles

Try building a roof one level lower, maybe?
P.S. Just checked: building a roof works for creating a floor one Z level higher.