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It's not a structure that appears in your demiplane, it is the overall structure OF the plane.
This option exists so you can pick a theme for your plane
If you pick "mountain" that means people in your demiplane are on a mountain, not they see a mountain. The size limitations and borders still apply.

Mysterious Stranger |

Considering it has a casting time of 4 hours and a duration of 1 day per level this spell is not something that is casually cast. Most casters that are capable of casting it will usually have a casting stat high enough to gain at least one more casting per day. At 15th level (minimum level to cast the spell) you can create 300 10 foot cubes per day. At that rate it will not take long to create a decent size plane.
When you are adding features to a plane you add the feature to the entire plane. This allows a caster to spend a couple days creating the plane and then add a structure or other feature with a single casting of create demiplane.