I understand the supernova process creates the heavier elements.

And, I understand supernovas explode mostly in all directions. Radiation may be more directional, and the mass is spread in basically all directions. I would imagine it would keep going until it was affected a bit by other material and radiation, but in a very short time there would be only a diffuse cloud which was busy becoming more and more diffuse.

How does enough iron, for instance, ever congregate in a single area long enough to be swept up into another star and planets?

Just curious. I'm sure there's an explanation.