Different nuclear fusion reactions produce different nuclides. Most of the atoms present in the universe are a result of stellar chemistry. Essentially pushing atoms together under extreme pressure and temperature so that the nuclei fuse into a single nucleus. All the other elements and their nuclides after these elements formed as a result of nuclear fusion inside stars, and supernova after the death of very large stars. Shortly after the big bang, the vast majority of atoms were hydrogen, with a little helium and lithium too.

We have to think back to how atoms were created in the first place. So if you have a molecule with a molar mass of 10.00 g/mol then what it's telling you is that if you gather together a moles amount of those molecules, 6.02214076 x 10^(23) molecules, then it will have a mass of 10.00 grams. Molar mass is usually given in units of grams/mole, or g/mol. Molar mass is the amount of mass contained within a mole amount of a certain substance. This 6.02214076 x 10^(23) is also known as Avogadro's number, or Avogadro's constant. Instead of saying I have 6.02214076 x 10^(23) atoms in this beaker, I can just say I have a mole of atoms in this beaker. Given how small and numerous atoms are we encounter them on the order of a mole often so it makes using it a convenient unit. So if you have a dozen atoms you have 12 atoms, but if you have a mole of atoms you have 6.02214076 x 10^(23) atoms. A mole is officially defined as 6.02214076 x 10^(23) particles, which is a very big number obviously. Similar to how a dozen of something tells you how many things there are, in a dozen's case 12. The mole is a unit for the amount of substance officially, or in other words how much of something there is. So certain combinations of protons and neutrons are stable, while others are not. This is why chlorine's isotopes, Cl-35 and Cl-37, are stable but Cl-36 is unstable and radioactive despite all three having a similar neutron-to-proton ratio. It should be noted that there is more nuance to this which includes nuclear binding energy. Unstable isotopes are also referred to as radioactive isotopes and prefer to decay into more stable nuclei. So stable isotopes have approximately the same number of neutrons and protons whereas unstable isotopes have larger differences in the numbers. Stable isotopes have a neutron-to-proton ratio of ~1 for smaller atomic number elements and approach ~1.5. So if atoms increase in the number of protons with increasing atomic number, they need a certain number of neutrons to provide enough stabilizing strong force. Essentially the protons and neutrons (collectively known as nucleons) exert a strong nuclear force on each other which counters to repulsive force of the positive protons in the nucleus. Atoms need a certain neutron-to-proton ratio to be stable.

Molar mass on a periodic table