General Structure of the Demiplanes

What does the edge of a demiplane/non-infinite plane look like?
From the outside, a demiplane has no "edge" - it's a bubble of some sort, usually spherical, floating in the Deep Ethereal.

From within, the specific nature of the demiplane determines that. It may be that the "edge" is a shoreline, or an infinite wall. It could be above the clouds in the skies surrounding a planetoid. It could be a door at the base of a stairwell, or checkerboard tiles breaking apart and spiraling slowly into a starry endless void. What the space "beyond" a demiplane looks like is as much a quality inherent to that demiplane as anything else.