If this melanin is high, the skin becomes darker, and if it is low, it becomes whiter. In other words, dark people have more melanin.
This melanin is usually spread over the skin on average, but sometimes it clumps together in one place. These dark clumps are called moles.
However, it is not yet known what causes them to cluster in one place.
A similar type of mole is "freckles," which are said to be inherited, but the details of this phenomenon are not yet known. There are many things about the human body that are still unknown.