How are elements heavier than iron, such as gold and uranium, primarily created?

Through intense conditions like the r-process during a core-collapse supernova.

While a star can efficiently forge elements up to iron ($ ext{Fe}$) through standard nuclear fusion processes, creating elements significantly heavier than iron, such as gold, silver, or uranium, requires an enormous infusion of energy and a high density of free neutrons. These conditions are not met during normal stellar fusion or even during the Type Ia thermonuclear explosion. Instead, the creation of these heavy nuclei primarily occurs within the extreme environment of a core-collapse supernova. Specifically, the rapid neutron capture process, known as the r-process, happens when the collapsing core forms a rigid object and the outer layers rebound violently, creating a massive shockwave and intense neutron flux necessary to build these heavy isotopes.