1Czar
Simple mobs spawn through eggs. Often these can be taken in a creative. Chickens can be spawned thanks to the eggs they carry (but normally spawning a chicken is extremely rare). So mostly neutral mobs like cows, chickens and pigs will spawn others if you feed several of the same species. The rest, including hostile ones, are produced by special spawners in the form of cells. They can be found underground. Exceptions are Iron Golem and Wither. They are done according to the scheme below.
Golem
Spoiler 
White - iron blocks. The one above is a pumpkin. Wither
Spoiler 
The principle is as follows:
Small objects on top are the heads of wither skeletons.
Below are the sands of souls mined in hell.
1Czar
It is easier to make them in creative mode by throwing eggs into the desired area to spawn.