https://www.forbes.com/sites/startswithabang/2018/06/13/the-...
although it did not explain how black holes form then. do they need a sufficient density of bosons? do those only form in supernovae?
No. Even degeneracy pressure from the Pauli exclusion principle has a limit; it can't hold an object up against gravity for an arbitrarily large mass. So neutron stars, like white dwarfs, have a maximum mass limit; any object over that mass cannot be a neutron star because neutron degeneracy pressure can't hold it up against its own gravity. (The article briefly mentions this; the limit is the Tolman-Oppenheimer-Volkoff limit.)
We don't know exactly what that maximum mass limit is for neutron stars, because we don't know their equation of state with sufficient accuracy (whereas we know the white dwarf equation of state accurately enough to know that the maximum mass limit for them is 1.4 times the mass of the Sun), but we know it's somewhere betweena about 2 and 3 times the mass of the Sun. Objects more massive than that can only be black holes.
As far as how black holes form, they form when a sufficiently massive object collapses under its own gravity. Usually this happens when a star runs out of nuclear fuel. (More precisely, this is how black holes of roughly stellar mass form. For supermassive holes, like the one at the center of our galaxy, things are more complicated, because they probably don't form in one single process, they form over a long period of time as a hole that started out a lot smaller swallows other massive objects.) If the collapsing object is over the maximum mass limit for a neutron star, degeneracy pressure from the Pauli exclusion principle cannot stop the collapse.
There is a theory that black holes can form from direct collapse. In this case, as I understand it, it can skip the star phase and just collapse straight into a black hole.... but I really am not an expert.
You can find more here: https://en.wikipedia.org/wiki/Direct_collapse_black_hole
And I HIGHLY recommend the PBS NOVA episode "Black Hole Apocalypse" (Jan 10, 2018, Season 45 Episode 1). It was on Netflix for a while (and may still be.)
Edit: here is the full episode on PBS (thank you NOVA!!!) https://www.pbs.org/video/black-hole-apocalypse-yj34qi/
One (pedantic) correction: objects more massive cannot be held up by neutron degeneracy pressure, but you can have more massive stars held up by radiation pressure from fusion. Certainly, there are many giant stars in the 2-8 solar mass range, and rarer stars in tens-to-hundreds of solar masses. Of course, they will eventually burn away their fusion fuel, and the core remnant may indeed become a black hole.
Radiation pressure actually isn't significant in the structure of most stars (it might be for huge stars near the Eddington limit). What holds up stars is ordinary thermal pressure due to the high temperatures inside them. The high temperatures are maintained by the fusion reactions in the stellar cores, yes. You're correct that objects like this do not have a maximum mass limit the way white dwarfs and neutron stars do.
[0] https://en.m.wikipedia.org/wiki/Kugelblitz_(astrophysics)