Hello Michael,
That’s a common feature in just about any 3d program. Since you won’t ever really enter a cube it only renders one side of the cube (the outer portion) to save on performance. If you were to zoom in on a mesh in Blender it would “clip” and would look invisible inside, it’s the same technique.
Same is true for just about any other 3d object you’ll create. If you wanted to see both sides of the mesh rendered you’d need a special shader, or if you were creating the mesh in something like Blender you could model it in a way that would display faces from every angle you expect to see it from.
If you’d like to try out some shaders you can try this free one out: https://assetstore.unity.com/packages/vfx/shaders/double-sided-shaders-23087
