How are elements heavier than iron primarily created in supernovae?

Via the rapid neutron capture process (r-process) during the collapse.

The synthesis of elements throughout a star's life follows distinct stages dictated by temperature and pressure. Lighter elements up to iron are produced through sequential fusion steps. Elements like Sulfur-34 are formed during general fusion, and those up to Nickel-56 are forged during silicon burning near the end of a massive star's life. However, the creation of elements heavier than iron requires an extreme influx of neutrons that standard fusion cannot provide. This process occurs during the catastrophic core collapse phase of a massive star, specifically through the rapid neutron capture process, known as the r-process. This environment provides the necessary high neutron flux to rapidly build up heavy nuclei before they have time to undergo beta decay, ensuring the creation of elements such as gold and uranium, which are then distributed into the interstellar medium via the supernova explosion.