Question

How does a black hole occur?

Answer 1

Massive stars after burning out most of hydrogen or other elements makes gravitational collapse ans shrinks to very small size.

Explanation:

Surface gravity depends upon mass and distance from center .When n huge stars collapse, the surface gravity become so large that the even light can not escape form it ids called black hole.
There may be some of them left over from big bang also.
Picture credit picture about space.com.