Black holes are the most terrifying and strange celestial bodies in the universe. What we usually call black holes refers to stellar black holes. In the process of death, massive black holes will collapse sharply inwards and eventually form black holes.
It takes a long time to grow a supermassive black hole, even if it eats voraciously. So just how supermassive black holes billions of times heavier than the Sun formed within the first billion years of the universe has been an enduring puzzle.