[roywiggins]

It's all squished into the black hole and from the perspective of everyone outside, converted into... more mass of the black hole. The mass of the black hole comes from the mass that created it. As you feed it more stuff, it gets more massive.

As to what physically happens to the stuff once it's inside, I don't know if we know for certain. It gets dragged towards the center. From the point of view of the rest of the universe, it might never actually reach the singularity: GR would make it look like it's going slower and slower and slower.

Speculation about what is actually inside the event horizon is at most mathematical extrapolation, since we can't actually crack one open and look.