Think of it this way. A Neutron Star is sitting right at the precipice of 3 solar masses. And you add just enough mass for the radius to fall below the event horizon. For all we can tell, this looks like a black hole. However, all that has really happened was that the event horizon (which was previously inside the Neutron Star) is now outside the Neutron Star. So what is physically happening here? Was there a Black Hole singularity inside the Neutron Star the whole time? Just waiting to emerge? But then wouldn't the Neutron Star just collapse as soon as it formed due to a lack of neutron degeneracy pressure in the core. (or maybe Neutron Stars cores are just collapsing really slow from the outer layer's perspective? See below)
I think perhaps the video may have been unintentionally misleading in the part leading up to the 'emergence' of the event horizon. Before the final collapse, there existed no 'real' event horizon; what was being depicted was the the Schwarzschild radius for the mass of the neutron star, ie. the volume its total mass would need to be compressed into for an event horizon to form. At the moment where the star's mass and density increase to the critical point, where the Schwarzschild radius is equal to (or greater than) the star's radius, a 'real' event horizon promptly appears.
There probably are circumstances where a region inside a larger object can reach that critical mass/density limit and collapse, then proceed to eat the rest of the object; in fact, I think this is theorized as the source of one type of gamma-ray burst. I
think, however, that neutron degeneracy pressure prevents the inner regions of a neutron star from attaining a high enough density until the whole thing reaches the
TOV limit.
Ok now wait for the appropriate time for that small lump of mass we added to re-emerge via hawking radiation. Now the mass is reduced back to that of a stable Neutron Star, but does the Neutron Star also re-emerge because it was sitting just inside an intense curvature of spacetime? Or is it now a fully fledged black hole just because the Even Horizon moved? This feels like an arbitrary condition that is based more on the observer more than the nature of the object, and this is why I suspect it is wrong, and that the Neutron Star was there all along. Granted that the the rate of infalling of the old surface of the Neutron Star towards the singularity could exceed the radius reduction from Hawking Radiation, so that even though the radius of the Event Horizon has fallen in, the surface of the Neutron Star has fallen further and is now still just below the smaller Horizon.
But then this means that the core of all Neutron stars are really Black Holes that are collapsing on apparent timescales so long that the degeneracy pressure is still felt by the layers above it for eons. Which is consistent with what we think we know is happening inside, but also feels like a mental bandaid.
Well, from the outsider's perspective ('at rest' with respect to the black hole) the collapse never actually finishes; the final instant is frozen in time and redshifted beyond all detectability on the horizon, so nothing* can actually emerge from it again. From the infalling point of view, spacetime gets kind of messed up once you cross the horizon (although the observer wouldn't really notice). The curvature at the horizon is so great that all possible paths anything can take inevitably lead to the singularity. And with nothing able to prevent the further collapse of the infalling matter a singularity forms, effectively at infinite density.
Bear in mind I'm no astrophysicist, so this is all 'interested layperson' levels of accurate.
Amusing thought: if you magicked into existence a sphere the size of our solar system filled with air (at standard, sea-level pressure), it would be a black hole. Big black holes have surprisingly-low density.
* For quantum-mechanical values of 'nothing'.
Hawking radiation is
weird.
