Extra buildings to build brick by brick, and small mods that make building easier. Every download contains the same unmodified BepInEx 5.4.23.5, so the mods work together: unzip one after the other into the game folder and overwrite.
Slots: the Taj Mahal is played in world slot 3, the Notre-Dame in world slot 4, each in a new save there.
Unofficial fan mods, not made by the game's developers. Ladder, SkillPoints, SkillModify and BuildHelper's ghost hand mark a world as practice (no Steam leaderboard score) once they change something in it; the Extra Buildings worlds are separate from the main world.