While Playing in a flat world i discovered that structures from every biome were appearing in the world (which was a plains flat world) . 

the screenshot attached shows an igloo, jungle temple, witch hut, shipwreck, and ruins.

the preset used in the flat world generator is below:

minecraft:bedrock,70*minecraft:stone,5*minecraft:dirt,minecraft:grass_block;minecraft:plains;village,biome_1,mineshaft,stronghold,decoration,lake,lava_lake,dungeon