As for black holes, they are composed of giant star evolution, black hole merger and so on. Let’s talk about how late stars form black holes.
when a massive star evolves into a red giant star, the fusion reaction inside the star stops at the iron core, and no new fusion occurs. Without the release of energy, there will be no outward radiation pressure. Gravity will prevail, causing the star to contract sharply inward. When the outer material shrinks inward and approaches the iron core, the speed may even be close to the speed of light, but the iron core is very strong and hard, like a wall, bouncing matter back at almost the same speed.
when matter collides with the iron core, it not only brings strong kinetic energy to the iron core, but also makes the matter rush out of the star in the form of internal explosion, forming a supernova explosion. This sustained impact caused the supernova to explode for a period of time. In addition, when the external matter impacts the kinetic energy input of the internal iron core, the internal matter and iron core are continuously compressed, the volume decreases and the density increases. According to the law of gravity, its gravity becomes larger. When gravity is so strong that its own light is attracted by itself and cannot escape, the core of the star becomes invisible. This is called a black hole.