How are black holes formed?
Black holes are formed when the core of a massive star collapse to within its Schwarzschild radius.
When a star of more than eight solar masses get to the end of its life it has an Iron core. Iron fusion is not possible as it requires more energy than is released. Once fusion stops the core starts to collapse.
When the pressure increases it exceeds the electron degeneracy pressure and atoms can no longer exist. Protons and electrons combine to form neutrons. This process causes a supernova explosion leaving a neutron star.
If the mass of the neutron star is more then 5-10 solar masses then neutron degeneracy pressure is exceeded. This means that neutrons can't remain separate and degenerate.
Once the collapse makes the radius of the core remnant less than its Schwarzschild radius it becomes a black hole.
The Schwarzschild radius
Any mass which is squeezed into a radius less than the Schwarzschild radius is a black hole.