The infrastructure for slavery in Africa was already well established by the Arabs. Black people were trafficked from the interior by the Arabs for centuries prior to European arrival. Arabs laid the foundation for Europeans to come and straight up copy them.
In fact, until this day, the common word for “Black person” in Arabic language is Abeed. Abeed literally translates to Slave. Arabs enslaved blacks and so considered all Black people slaves that it became their literal name in Arabic language. There is actually a movement now in the more progressive Arabic communities to stop casually calling Black people “Abeed/Slaves” in their language.
The Euros saw how lucrative slave trading was for the Arabs and they straight up copied them. They needed people for the new world since the people already there were all dying from smallpox and torture. Plus they were greedy af.
Arabs had different “uses” for black people. They would make the males eunuchs and the females sex slaves for harems. That’s one reason why you don’t see the descendants of these slaves as much in the Arab/Indian regions. They were absolutely chained up, dragged to the coastal ports, and brought to Arabia/India on boats called dhows.
Source: spent a significant time in Zanzibar, learned the history there. I highly recommend traveling there and going to the old slave ports and seeing and learning it for yourself from the people and guides there. They explain detailed things about their history you will not find online from Western sources.
East Africa’s forgotten Slave Trade
This 1973 paper written by a researcher at a Swedish university containing an extensive bibliography
Another source- a book I bought in Zanzibar:
Dhow chasing in Zanzibar Waters and the Eastern Coast of Africa - a memoir by a captain in the late 1800s who stopped dhow slave boats in the Indian Ocean (East coast of Africa and Zanzibar)