How are elements heavier than iron, like gold and uranium, primarily forged in the cosmos?

Through rapid neutron capture, known as the r-process, powered by the tremendous energy released during the core collapse and rebound.

The production of elements heavier than iron, which includes very heavy metals like gold, lead, and uranium, occurs in environments far more energetic than standard stellar burning. This process is the r-process (rapid neutron capture), which happens in the highly energetic, non-equilibrium environment characteristic of a supernova explosion. During the outward shockwave resulting from the core rebound, atomic nuclei are bombarded by an intense flux of neutrons. These nuclei rapidly absorb many neutrons before they have a chance to undergo radioactive decay, thus building up the ultra-heavy elements in mere seconds. While iron itself is a product of internal stellar fusion, these heavier elements require the violence that follows the iron core's demise.