How do black holes form?

These days, they form from massive stars that become supernova. Their core regions implode, and no known force can withstand the force of gravity once the imploding mass gets close to its eventual size as a black hole; a size determined by the masses so-called Schwarzschild radius.

The cores of stars more than about 6 times the mass of the sun reach a condition late in their lives where the fusion reactions in their cores are producing such a high flux of neutrinos that the neutrinos carry away badly needed energy from the core. This causes the core to collapse and heat up as the reactions 'burn' at higher temperatures. This produces more neutrinos and the process runs away. As the core collapses, the density increases so high that suddenly the neutrinos can't escape, and within a few hours the bottled up energy causes the star to detonate. This produces a reversed shock that compresses the core matter beyond its mechanical ability to support itself. When it reaches a size equal to the black hole horizon size for that mass, it becomes a black hole as it explosively sheds the rest of its mass in a supernova. The picture below shows the supernova explosion in 1987 which very likely produced a black hole.