This is a thread just for you two in which we will, if you dare to reply at least, go over this topic step by step.
If you find some issue in a step, please ONLY reply to that issue so we save space and can go over it thouroughly.
0)
As a preword: Every possible approach to mathematics (and there is more than one) needs axioms, so statements which we simply see as true.
If we do not at least assume something exists, like the empty set, or the natural numbers, or anything like that, we cannot do math simply because we will never be able to prove that anything of the things we talk about exists.
The only thing that is important about the axioms we have to choose thus is that they a) do not contradict each other and b) are useful.
For example, the axiom 'the empty set exists' alone would be completely useless, because that would be the only statement ever provable in that math.
In ZFC, we have the axiom of infinity, stating that there is a set that contains the empty set and for every element x of it, also x U {x} is in that set (thus instantly making that set infinite, since that means there are infinitely many unique elements in it).
For the axiom of infinity, see here:
https://en.wikipedia.org/wiki/Axiom_of_infinity
Now, unless you can show me how the axiom of infinity contradicts any of the other set axioms, you'd have to accept that fact simply because it is an axiom.
You can say you don't like this axiom, but you cannot say it is logically wrong or whatever, because as an axiom, it is one of the things you simply have to choose as true to do math.
Even if we chose silly axioms like the axiomset that only states the empty set exist i mentioned before, as long as it is logically coherent, everything is fine.
So unless you can show me how that axiom directly contradicts anything else, let's go on.
1)
The first step is to show you that in fact the natural numbers are an infinite set.
That should usually go without saying, but i feel like you would debate that fact by flat out stating that no infinite set exists.
The natural numbers can be defined in the following way:
https://en.wikipedia.org/wiki/Set-theoretic_definition_of_natural_numbers
As you can see, the natural numbers have to exist due to the axiom of infinity (because they are the smallest set that satisfies the definition of the set constructed in the axiom of infinity, and thus as a subset of that set has to exist. This follows from the axiom of specification
https://en.wikipedia.org/wiki/Axiom_schema_of_specification . This axiom states that any subset of a set definable by a logical property (in FO) is a set. And the natural numbers are definable by such an FO property as being the smallest of the sets proclaimed by the axiom of infinity).
Now, all we did until now is using definitions and axioms.
So we only used things that are definitely right in ZFC (and once again, unless you can show me a SETTHEORETICAL contradiction onf ZFC until this point, you cannot deny their truth) and gave names to some things we know exist.
By that, we know that the natural numbers exist and we know that they are an infinite set (since we defined it as the smallest set satisfying the definition of the axiom of infinty and that set has to be infinite since it has to contain infinitely many unique elements per definition).
2)
Now, we define cardinals. We have the natural numbers at our disposal now and define the cardinality of a finite set as the number of its elements.
That is very unproblematic.
However, we know that the naturals are not finite.
There cannot be any finite cardinality we can give them since it would contain the corresponding number and the sequence from 1 to the successor of that number would be a set with a bigger cardinality than the number we wanted to assign the natural numbers.
By now, we simply know one thing: The cardinality of the naturals can NOT be finite.
However, we wish to extend our cardinality definition to infinite sets.
And that is not hard. We can simply call the cardinality of the naturals aleph_0.
Whether this makes sense or not is a secondary question, because we may.
There is nothing stopping us. Naming a thing is NEVER wrong in math.
You could name the set that contains all others sets biggle boggle, and it would not be a problem.
Such a set does not exist in ZFC, but that simply means that biggle boggle does not exist and nothing is wrong or lost by giving that nonexistant set a silly name.
So, we call our cardinality of the naturals aleph_0.
Aleph_0 can not be any natural number because we already verified that the naturals are infinitely big and thus cannot have a finite cardinality.
However, we wish to compare different infinite cardinalities with each other.
Thus we give cardinality a braoder definition: Two sets have the same cardinality if and only if there is a bijection between them.
That holds true for finite sets in any case, and here we can, once again, see, that a finite set cannot have the same cardinality as the naturals do because there can never be a surjective map from any finite map into the naturals.
3) With our definition, we just have to check whether two sets are in bijection with each other.
For the rationals and naturals, that is true.
We can create a list of all rational numbers with cantor's diagonal scheme (we go 1, 1/2, 1/3 ,2/3, 1/4, 2/4 and so on going through all proper fractions) and thus we know that the cardinality of the rationals must be aleph_0, thus the same as the one of the naturals, BECAUSE THEY ARE IN BIJECTION.
Now, there are two good ways to show that not all infinite sets are in bijection with each other: The power set of the naturals and the reals.
I know that you take issue with real numbers and them being infinitely long, so i decided to give you the proof for power sets:
https://en.wikipedia.org/wiki/Cantor%27s_theorem
I can explain it with some more explanation if you wish to, but this proof is logically sound and does not have anything to do with real numbers.
However, with it we can show that in fact the cardinality of the power set of the natural numbers is strictly bigger then the one of the naturals.
Thus we see that in fact it makes sense to define cardinality with bijections.
We can clearly show that this seperates finite sets from infinite ones and it does seperate even infinite cardinalities from each other in a way that is useful for quite some logical applications.
_______________
So, that is it. You can ask for further explanations or try to find contradiction in that if you want to, but there is really nothing done here except using axioms and definitions.
The only proof is that a power set is bigger than the corresponding set, and you would also have to find a mistake in that proof before you can claim anything here is false.