How do workers prepare to make a bridge?

im asking about bridges that go over like a river or some time of body of water. how do they prepare the major bridges, specifically the beams that hold them up, if all of the water there is in the way? im guessing they transport in the concrete slabs and everything to put as a base foundation for the pillars that hold up the bridge, but what if the bottom of the river floor is uneven and everything? how do they prepare that?