Black holes appear when a supermassive star stops producing energy and collapses under its own weight. It becomes so small and it is so heavy that light does not go fast enough to escape from it. so we can’t see it anymore!
I’m not a cosmologist but learned during my undergrad physics degree the basics of them. The extreme conditions under which they form and how they behave is are basically understood, we think. Black holes have mass, charge and angular momentum and the rest depends on the material outside the event horizon.