When I was a child, the friends who had chickens in the family must have seen the scene of the rooster bullying the hen.
Moreover, when I was young, I had a sense of justice, and when I saw the rooster riding on the hen and pecking at it, I would always pick up a stick or a stone to help the hen and drive the rooster down, and at that time I felt that I had done the right thing, and I had a special sense of accomplishment. To this day, I regret it too much, and the past is too much to look back on.
Later I learned that it turned out that the rooster "stepped" on the hen, this process, that is, mating, commonly known as "pressing eggs", the rooster from "stepping" on the hen's back, to the down, that is, the mating time from the beginning to the end, the whole process does not exceed 5 seconds.
The hen is certainly complete before the egg is laid, the rooster is not like a human, has protruding extracorporeal reproductive organs, he looks similar to the hen, is a cloaca hole, and of course there is a little bulge.
During mating, the hen's tail spreads out, revealing the cloaca, and the penis inside the cock's cloaca expands into a circular bulge (very short) that protrudes from the cloaca and comes into contact with the cloaca of the hen. At the moment of contact, the cock's "outflow (not ejaculation)" sperm enters the hen's cloaca, while the rooster's tail is covered downwards (which is why we don't see the chicken's sexual organs).
As for the cloaca of chickens, the following picture gives you an at-a-glance look:
Comparison of the penises of chickens, quails, ducks and geese, the scale in the figure is 1 cm.
After the egg is formed, that is, after the egg yolk is formed, it will be inseminated with the stored semen, then wrapped in egg whites, and finally the shell of calcium carbonate will form an egg.
Roosters produce 1.5-8 billion sperm per mating (400-600 million in humans). Sperm can survive in the hen for 35 days, so this is why some eggs, after laying, as long as the hen hatches, it can hatch into chicks.