You almost have it, but then it fails when you jump up a dimention. The surface is every direction.

By flat, they don't mean like a sheet of paper. They mean flat in that parallel lines never cross.

You are at the center of the universe right now, and so is everything else. It is all there ever was, just spreading apart, (but not from a central point), but still the same thing with more room to do stuff.

There is no outside or inside. The surface in 3D is all there is (plus time).