How come a billion of people are born just from Adam and Eve, does it mean that an incest was going on between the first people on this planet?

The truths of divine revelation cannot be deduced through natural
reason.

The primary purpose of the way the story of Adam and Eve is constructed is to show that each of us as humans were all represented by the first Human---Adam, from which all of us "came."

In this case, I find it is useful to go with the the general concept and point of the story, rather than trying to figure out the details of how it might have worked.

Short answer: Yes. Regardless of how you believe humanity has flourished, the modern definition of "incest" would have played a major role in reproduction.

Circumstances were quite different in biblical times. Our first parents Adam and Eve were created perfect, and the instructions for them to "multiply and fill the earth" would necessarily require that their offspring marry one another and produce. (Genesis 1:28)  Therefore, as perfect humans, their children would have been perfect as were their parents.

Even though Adam and Eve sinned and became imperfect, Cain and his brothers and sisters were still so near to physical perfection that the children they produced did not suffer the same adverse effects as do children born of such unions today. Even some 2,000 years afterward, God’s faithful servant Abraham married his half-sister Sarah, and God did not disapprove. (Gen. 20:12) It was yet another 450 years or so before God saw fit to provide his nation of Israel a body of laws that forbade incest on penalty of death. (Lev. 18:8-17) By that time imperfection had apparently developed to such an extent that no longer was it safe for close relatives to marry.

Very interesting question here as basically it means that we all are related because of this.