After reading Genesis several times over many years I realized that something was totally missed in the very first chapter...

God made Mankind and gave them dominion over all Creation. He told them to be fruitful and multiply and to replenish (to fill) the Earth.

That answers the question of the suppose conundrum of Cain's wives I should think...

Still and all, I'll go further...

Afterwards, God set aside some protected property, made a Garden Paradise, and created Adam and Eve to tend to its care. He also told them to be fruitful and multiply...

By the way, that story is the basis for the concept of "holy" which simply means "set apart" - and in the context of all following Bible stories God "calls people out for His purposes." In both Old and New Testaments this concept is reinforced: "Wherefore come out from among them, and be separate, says the Lord, and touch not the unclean thing.". This principle is found in both 2 Corinthians 6 and Leviticus 11.

I'm guessing that for educational purposes ,for children's benefit, someone in charge decided to make the Bible micn simpler and more age appropriate, and so, the facts of the first chapter were lost. The whole was cut in two...

We have also lost perspective of the consideration of the length of the span of time in which all these events took place.