They used cheap plastic drywall anchors, the kind that split into two small “half-rods” (for lack of a better term) when you screw in. The problem is the anchors are plastic and the plastic bends pretty easily, so the load isn’t really distributed much at all. If you look at the holes near the bottom of where the shelf was that’ll give you an idea of how little they spread out.

As for how much stuff, a handful of boxes with things I access semi-frequently, mostly game controllers, network cables, extension cords, so on. No books or anything. Far less than I’ve put up in other places I lived at actually. More to the point, far less than what the previous occupant had up there (we helped move out her stuff).