Answer to Question 1
With each generation of supernova, the interstellar medium is showered with a larger abundance of the heavy elements forged in the giant cores and blown back into space with their explosions.
Answer to Question 2
Iron is the most tightly bonded of all nuclei. Fusion reactions leading to its production release radiation pressure to resist gravity, but all heavier nuclei beyond iron will absorb energy in their formation, so without the radiation to keep it inflated, the core collapses rapidly.