Wheat Plains are biomes containing three mobs: Sheeats, Chickens/Roosters, and Ostriches. They are flat lands, with full grown wheat, grass, some flowers, ferns, water, and rarely, lava pits.
Wheat Plains can be found very rare on the map, however, it is only recommended to search for one if you are looking for fully-grown wheat. You can NEVER spawn on one directly, however, you could spawn beside one.
There are various minerals found directly under the grass, such as Coal, Iron, and Stone, meaning, there is no second layer of dirt and grass.
Villagers sometimes can be seen here, and harvest some seeds and wheat. However, you can kill it, and gain the wheat and seeds back