After the big bang, protons, electrons, and neutrons were formed through

As the space expanded, big bang happened and resulted to the formation of protons, electrons, and neutrons through nuclear fusion that scattered like fog in the universe. During the fusion, the sun and other stars generate heat and light. The light elements break up into dense plasma and the temperature cooled, which form matter.

In addition, nuclear fusion create a reaction in which two or more atomic nuclei are combined such as protons and neutrons collided to make deuterium, an isotope of hydrogen and when deuterium combined, it will make helium.

 Nuclear fusion

hope it helps :D