Question

where do atomic mass come from?​

Answer 1

Almost all of the mass of an atom (more than 99%) is in its nucleus, so "atomic mass" is pretty much a measure of the size of the nucleus of an atom. The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size.

Answer 2

The atomic mass is made up by the mass of protons and neutrons as the electrons have negligible mass.

Hope this helps!