The lighter elements are made up in the fusion process that exists in the core of the stars. What about the heavier element?

There are a few little neutrons and an iron nucleus. As the seed nucleus is bombarded with these neutrons, it swells and turn into a much larger nucleus, becoming both radioactive and neutron-rich, ultimately qualifying it is an isotope.

Because it is radioactive, it does not like to stay in this way. In fact, it with all the neutrons are converted into protons, leading to the formation of a stable element that is much larger than the original iron. It could be a carbon atom or uranium.

An example of uranium 238 is technically not a stable element (half-life is 4.7b years). For humans, it is stable but on cosmic timescales, it is not.

The above process so-called neutron capture process. There are 2 ways of neutron capture process: slow n-capture and rapid n-capture. They refer to how fast what timescale this neutron bombardment is occurring.

For the slow n-capture, the timescale is about 10,000 years. Elements produced by this process can contribute to the evolution into a red giant stars. The n-capture process happens for many times until the heaviest element is formed, called Pb.