Any given plane, unless you make a double sided shader, will only show from one side. This is actually by design.
The easiest solution is to make both sides. Instead of a single plane for a side, you would need an actual thick cubic kind of thing, something with thickness. Then no matter which side you are looking from, you will see something. It works the same in real life. The pieces of wood that make that table have some thickness anyway, and you could look at things from the top and see that thin edge as well. A single plane can’t do this.
The difference in what you see in blender and unity are due to the differences in the shaders. Specifically blender does not cull faces while unity does. When face culling is off, like in blender, both sides of the face are drawn which is better for editing. When the back of faces are culled, like in unity, only polygons that face the user are drawn, which is better for performance. So you need to either make two polys where you want a two sided polygon or use a shader with Cull Off.
Are you using just 1 polygon face for your walls? If so that is your problem. Make a box, make it very thin and use that. Either that or a faster method would be to clone the 1-polygon walls, then flip the polygons by right clicking and selecting flip faces or flip normals…