The formation of black holes or neutron stars after the collapse of massive stars is determined by their initial mass and other factors. While not all collapsing stars turn into black holes, those with sufficiently high mass are more likely to do so.
When a massive star exhausts its nuclear fuel, it can no longer generate the outward pressure that counteracts gravity. The core of the star collapses under its own gravity, resulting in a catastrophic event known as a supernova. The core's collapse is driven by the force of gravity, causing the outer layers of the star to be expelled in a powerful explosion.
The outcome of this collapse depends on the mass of the core. If the core has a mass below a certain threshold known as the Tolman-Oppenheimer-Volkoff (TOV) limit, which is roughly 2-3 times the mass of the Sun, it can be stabilized by the pressure of subatomic particles, primarily neutrons. This stabilization leads to the formation of a dense object called a neutron star.
However, if the collapsing core's mass exceeds the TOV limit, the gravitational forces are too strong for the neutron degeneracy pressure to counteract them. In this case, the core continues to collapse, reaching a state of infinite density known as a singularity. The region surrounding the singularity forms an event horizon, beyond which nothing can escape, giving rise to a black hole.
So, in essence, the formation of a black hole or a neutron star depends on the mass of the collapsing core. If the core's mass is below the TOV limit, a neutron star is formed, whereas a mass exceeding the limit leads to the formation of a black hole.