[fluffkylass00]

I understand the difference between atomic number, mass number and atomic mass. however, I do not understand the difference between relative/average mass ( which is weighted average of all isotopes of an element) and the atomic mass found out through comparison with the mass of one atom of carbon-12. in the earlier, we are not comparing anything to carbon-12 atom. I don't understand what is happening in the latter mentioned. how are we finding the atomic mass( what comparison are we doing and how) and is the atomic mass that we gain by comparing to carbon-12 atom ( however that happens) the same as the average/ relative atomic mass( which I see being mentioned in the periodic table). are they different? are there any other kinds of atomic mass and ways to find them, besides the ones I've mentioned?

[Borek]

Does the fact an element - as it exists in a nature - is a mixture of isotopes, help?

When we deal with large samples of substance (which is form most practical purposes always) we deal with mixtures, so the mass of a single atom doesn't matter. What matters is the observed mass, one that we can experimentally check by weighing the sample and counting atoms in it (by whatever means).

Unit used, and the way it is defined, is secondary. We just took some mass (that was in some way convenient to define and use, but that's irrelevant) and said "from now on this is the standard mass we will use".  It is like with using feet, or meters, or inches, or yards, or miles - it really doesn't matter which you use when comparing distances, as long as you are consistent and express all distances using the same unit.