You've heard the saying that mass and energy are pretty much the same. However its not exactly true, but close enough for the average person to wrap their head around. The thing about black holes though is energy density. When enough energy is packed into small enough pockets of space, that energy sinks further and further, faster and faster into the well its creating. Just as a planet or star has a gravity well. Black holes follow the same general rules, the differences however are the extreme effects such a well has on the very space around it. Just by its rotational speed it can twist and wrap itself in the space around it. Now the energy portion equates to energy density yes, and every atom has bound energy that bonds the atoms, electrons, quarks, etc together. Heavier the metal, the more energy that is contained in the bonds holding it together, again coming down to neutrons, electrons, etc. Energy is energy, whether massless or not. I am sleep exhausted so I repeated myself a bit and didn not explain very well, but I hope it was well enough to convey a better understanding. If you wish for a deeper explination, I remember PBS spacetime had an episode that explained it much better than I can, sleep exhausted or no.