Stellar element synthesis is where most elements, iron and below, form. Hydrogen, the most common element, fuses to Helium, Lithium. There are more cycles to stars burning elements, Carbon-Nitrogren-Oxygen Cycle, and a bunch of other stuff, all the way up to Iron. After Iron, nuclear fusion can no longer sustain the star, and it collapses into a neutron star (or any other intermediary ranging from hypothetical quark stars to black holes).

On collapse, you get a supernova. Supernova and other high energy events (called Gamma Ray Bursts, usually attributed to Supernova anyway) explode in a shower of neutrinos and gamma rays. These neutrinos rarely interact with matter since they have no charge, but they still contain a lot of energy, traveling near the speed of light. Gamma rays are the highest energy photons. Anything either particle interacts with will change it.

The collision of the gamma rays burst and nuetrinos with interstellar matter creates the remainder of the elements, much in a similar way we do on earth to create the synthetic elements (like plutonium).

Any isotope can be created this way. Isotopes that are unstable then decay until they become something stable - Uranium -> Lead.

The universe is so old that enough of these elements were able to gather by gravity, forming the relatively tiny deposits we can find on our planet.