where does all this debris come from?

Doesn't say, but there's a lot of junk up in orbit, paint chips from things put up there, bolts, micrometeorites, things like that. Some is man made and some probably is not.

"Space debris" usually refers to man-made stuff, anything from defunct satellites to flecks of paint. There are natural things like micrometeorites around as well. The big problem is the more pieces of debris you have, the more collisions are likely to occur, and those collisions produce yet more debris.