Adam and Eve got many children and their children also gave birth to several children, so that the world was populated as God had told them.
Kayin of Cain is more concerned what would happen to him after he did that terrible act (of killing his brother). God, instead of making an end to Cain's life straight ahead, left Cain alive as an even more poignant sign of sin.
What the Scriptures tells us about how God reacted on Kayn (Cain) his act of offering and killing his brother
We come now to the position that the 1st man had relations with his wife Eve, and she conceived and gave birth to Cain, who when older brought offerings to please God.
The Word of God tells us how the first living souls became tricked by their own wrong thoughts and fell for the pleasing of their eyes and by their disobedience to God got punishments over them and got being banished from the Garden of Eden.
In the second chapter of the Bereshith light is shed on the day that יהוה Elohim made earth and heavens and created a beautiful Royal Garden where man, made in the image of God, could live in peace, with the animals, to which he receives rulership (not ownership over the earth).
The Bible opens with the story of the beginning of everything, presenting the six periods of development (six days of creation) and the final period of establishment or rest.