Yes, pure math papers are a bit different. There is usually an introduction, which may give some background and also name the problem to be solved. There is the usual conclusion, recapitulating the important bits. But in the middle there are proofs of one or more theorems.
Since proofs can be long and complex, important parts may be broken out separately. Lemmas are such things - smaller scale theorems that have their own proofs but are intended for support of a larger result.
Corollaries are other theorems that, may have a lesser impact (or not) but which follow (fairly) easily from the main theorems.
Axioms, on the other hand are more difficult for the non-mathematician to understand. They are statements, given without proof, that form the basis of what follows. The proved theorems are true, then, only in the context in which the axioms are valid.
One of the most important standard axioms is: Zero is a Natural Number. When that is accepted along with a few other simple things, logic can be used to prove quite complex things. But those things might not be true in areas in which the concept of "Zero", or "Natural Number" make no sense.
I suggest that you build yourself a glossary of things like the above for future reference. One way to extend the glossary is to ask your students to explain organizational terms that you don't understand. They will probably benefit themselves by providing explanations.
A typical math paper might have sections like the following, in this (or similar) order, though there might not be actual section separations.
Introduction and Background
Problem to be Solved
Axioms (the basis)
Lemmas (with proofs)
Theorem(s) (with proof)
Corollaries (proofs if needed, some are obvious the reader)
Conclusion and Significance
Future Work
The lemmas and theorems might be one section or a repeated pair. Some definitions are also likely to be given, perhaps in a separate section or perhaps interspersed with other things so that they appear at point of first use.
One issue is that the methodology is usually just symbolic logic, formalized or not. That is understood to be the case so there is no need for such a section. And it is the major theorem(s) that is the result: statements that we can now take to be true given the axioms with the "evidence" being the proofs.
In some fields the axioms are understood from the field itself and so won't be stated. Important papers in which a new field is introduced (rare) would probably state axioms, likewise papers that deviate from common practice. Simple things may not need lemmas. Not everything has corollaries. And many papers have such a structure even if there are no headings to separate them out. And sometimes the headings are more specific, perhaps the statement of a lemma.
Also note that some other theoretical fields (say, theoretical CS or theoretical physics, even history) will also have paper structures tailored to the field, whereas IMRaD is more useful in the social sciences.
You can't expect to find the above headings/sections in any particular paper. They just represent a natural flow that can be seen if you look for it.
And, finally, some papers in applied math will have a structure something like what you are used to, especially as it may be important to make the methodology explicit.