The Mesopotamia was divided into 2 groups or kingdom.
The first one is the Sumerian, they are the people who spoke unrelated language to any modern language. And the other one is the Semitics, they are the people who spoke language related to modern Arabic and Hebrew.
Sumerians lives near the great river and built their first city in there.
While semitic became the dominant language of places like Akkadian, Amorite, Assyrian and Aramaic. So these are the region where semitic lived.