SIDH: key agreement - why does it work?
I invented SIDH and can surely answer any technical questions about it. Proofs of $E_{ab} \cong E_{ba}$ are given in several places, though rarely in official Theorem / Proof form, because this result is just an application of well-established theory on elliptic curves rather than any sort of new mathematical result that merits formal statement as a theorem. See for example the original paper (Section 3.2, in particular the (unique) displayed equation in that section), or De Feo's survey article (page 34), or Martindale and Panny's cryptanalysis summary (page 4, paragraph "Key exchange").

If you have a more specific question, please ask away.