There are relativistic effects that confine all infalling matter to the event horizon. From the perspective of an observer INSIDE the black hole, time moves faster. The rate of infalling matter accelerates to infinite, and all the matter in the outside universe is near-instantaneously sucked in against the event horizon, infinitely delayed there. But the mass of the whole black hole increases at the same time as a result, and so it gets bigger very fast. The 15-solar masses of material that created the black hole is stretched out until the interior of the black hole is almost empty, without curvature, cold. The black hole becomes a billion lightyears in diameter in an instant.

Soon the event horizon starts to evaporate Hawking radiation into the interior, in the form of virtual particles (baryonic matter), and gamma rays. The gamma rays are redshifted due to event horizon expansion, and after billions of years appear in the microwave range, hence the cosmic microwave background, or CMB. In my opinion, the following image is of the inner surface of the black hole event horizon.