When you create one of these world types: single biome/caves/floating islands, the generation will try and place their correspinding blocks and will not care about biomes.

single biome: stone is underground even if the biome is nether or end

caves: netherrack is underground and makes the roof even if the biome is overworld or end

floating islands: endstone is underground(bottom side of islands) even if the biome is overworld or nether