You appear to be using Rhino, so I would suggest that you search for differences between closed polysurface and open polysurfaces.
A closed polysurface represents a watertight geometry, whereas an open polysurface geometry is made of sheet parts. Your geometry is currently an open polysurface (see the right-hand side panel of the screenshot that you posted).
The model should be exported from Rhino as a single closed polysurface, representing the fluid domain. So you’re not really interested in the solid walls of the dome, but instead you are interested in capturing the fluid region inside of the building. You won’t need to go into CAD mode at all if you import a correctly constructed CAD model from Rhino.
Open polysurfaces, such as the model that you have, do not have any volume so they are not good for simulations, since they have 0 volume. You’d have to close gaps (if they exist) and join the faces to create a closed polysurface.
If these requirements or workflows are unclear, I would suggest that you post your question on Rhino forums.
Good luck!