Why was the Abbasid Caliphate seen as cosmopolitan?

When something is said to be cosmopolitan, it means that people from different parts of the world are habiting under one city, state, or city-state.

The Abbasid Caliphate was a long line of a family who had ruled Baghdad for more than 300 years and their reign was prosperous and brought developments in the Middle Age and several inventions that led to the Golden Age of Islam.

They are considered cosmopolitan because they had different nationalities living among them such as Arabs living with Indians, Turks, Armenians, and also Kurds living in a Jewish dominated area, and Christians living among Muslims.

from the conditions of the working class in england by friedrich engels,the main idea is :

"germany's geographic location was an advantage when it was on the offensive in the war because it was situated next to an extremely wide river, the rhine, which was difficult for the allies to cross."