How did the world become filled with different people, when it was just Adam and Eve in the beginning, who only had two sons, Cain and Abel?
The Bible doesn’t tell us about the daughters of Adam and Eve. It mentions the boys because genealogies follow the patristic side. But I assure you that at least Adam was in his 900s before she died. He populated the earth with many children.
Cain killed Abel before he could procreate. But Cain took his own place among the beginning of the nations. He separated from his parents and began his own families in a different part of the world. He created his own line (Genesis 4:17-26).
Adam and Eve conceived again and had Seth. From these lines the earth was filled with so many people that God eventually had to deal with the extreme wickedness upon the earth (Genesis 6:1). Because of the wickedness of humanity, human lifespan to 120 years gradually over time (Genesis 6:3).
We have no idea of knowing how full the earth was from the time of Adam to the time of Noah. In one sense, the amount of people from the line of Adam that filled the earth pre-flood doesn’t matter. God reduces the population of Earth to 8 people during the flood (Genesis 7:13).
Repopulating the earth begins again with Noah’s three sons. It is from these three lines that the earth was repopulated into different areas and regions. From these three lines we get all of the different human populations of the earth and all of the regions.
Based on person’s region, with different climates, you will get different -looking people. People closer to the equator, for instance, must find a way to resist the constant bombardment of the sun throughout every season. Over time they will become darker than those who endure longer winters where the sun is closer to the equator.
Each region takes on its own culture, language, and so forth. They may even develop separate from one another and have to find one another through exploration, trade, and travel.