I already understand, and so ask NOT about, the definition, but instead want to dig deeper:
if not = Perhaps even (used to introduce a more extreme term than one first mentioned)
How does (the juxtaposition of) if + not combine to mean the above,
which neither word alone means?
†This is where logic or mathematics operates differently from human language (aka natural language). Strictly speaking and mathematically speaking,
1. "N is Y, if not Z" would be read as
2. "if N is not Z, then N is Y".
However, in many human languages, English included, saying 1 doesn't mean just 2. To understand this, we should think a little about the motive of a speaker who said 1.
Quite likely, the speaker's intention was
3. "If N is not so great as Z (or any quantity > Y), it is = Y"‡.
But to say 3 exactly in those words would be clumsy,
and thus the speaker would naturally rephrase 3 as:
4. "N = Y if N ≠ Z",
or in plain English: 5. "N is Y, if not Z".
We can try another approach to show that if not can be used to mean perhaps even.
Let N is the number of people who has an attribute Q.
Consider these two following propositions:
(a) N is Y, if not Z.†
(b) N is Y, perhaps even Z.
Let Na is the set of all possible values of N for proposition (a).
It is easy to see that Na = {Y, Y+1, Y+2, Y+3, ..., Z}.
Let Nb is the set of all possible values of N for proposition (b).
It is also easy to see that Nb = {Y, Y+1, Y+2, Y+3, ..., Z}.
Now, it is clear that Na and Nb are equal. Hence, (a) and (b) are equivalent.
Thus, if not can be used to mean perhaps even.
Q.E.D.
Consider: X if not Y.
if not is used when you want to say that Y is false,
NOT because Y overstates the extremity of the situation, but because Y understates it.
if not is usually used with numerical values.
There are millions, if not billions, of bacteria on every inch of our skin.
This means:
There are millions of bacteria on every inch of our skin,
and if there are not, then there are billions (rather than thousands.)
If not effectively shortens the phrase if there are not, then there are.
I hope that's what you were looking for.
I gather you're talking about phrases like JasonPatterson's example: "There are millions, if not billions, of ..." It doesn't have to be numerical. "John is a smart guy, if not a genius." Etc.
The meaning would be plain if we reversed the order of the clauses and added some qualifiers: "If there are not billions, there are certainly millions ..." "If John is not a genius, he is at least a very smart guy."
That is, it's probably the "big" thing, but if it isn't, it must at least be the "little" thing.
But sometimes we want to give the little thing first, so that we build up to the big thing. I don't want to start out by saying that it's probably the big thing, and then have a let-down where I say, well, okay, maybe it's the little thing. So we turn the word order around and put the little thing first, then say, "if not ...", and then the big thing.
It's just a matter of re-arranging the words to achieve the desired effect. You don't want to give the punch line before you've told the joke.
