Inside the building you would see the white front faces.
The back face is the “inside” of building elements. You would not see the back faces at all in a completed model–for the most part, unless you are cutting into a wall or floor etc.
So the backfaces are inside the wall, but the front faces are the exterior and what you see of the interior of the wall (the wall has thickness–a space between the two faces).
A very simple building example. What you do where the parts of the building meet, like the foundation and walls, walls and roof depends on your method of modeling and your needs. I left no face there to try to simplify the example. in this case I made the backface default color red. A setting in the Styles window.
If, in fact, you were showing rafters and studs etc. the inside of the members would be back faces. Disassembling the house you’d see all front faces.