“Elements heavier than iron, up to bismuth, are primarily produced via the s-process (slow neutron capture) in low to medium-mass stars during their later evolutionary stages.

The remaining and heaviest elements (beyond iron and bismuth) are formed through explosive events: core-collapse supernovae generate elements between neon and nickel, while the r-process (rapid neutron capture) in supernovae and, predominantly, neutron star mergers creates elements like uranium and thorium, dispersing them into the interstellar medium for planetary formation.”

From https://www.astronomy.com/science/the-universes-guide-to-cre...