Why were Africans considered inferior or lower than white people?

Because of their skin color

In the medieval times, Africa was a populous territory and slavery was rampant there. Even before Europeans began colonizing various parts of Asia, Africa and America, slaves were being exported from Africa to Europe and Muslim countries. At that time, it wasn't the European domination or Islamic domination that led to Africans being enslaved. It was just that Africa had too many people and the kings of Africa found a way to encash that resource by selling it to the Europeans. Some exported African slaves were also soldiers taken prisoner during wars. So thats what basically started it my dude