If I attack attracting Azir, Sovereign with Moonfall, does it resolve first, Moonfall or Azir, Sovereign?

Yes — Moonfall resolves first, and in full. Azir, Sovereign's "When I attack" trigger isn't even put on the chain until Moonfall has finished resolving, because nothing new can be finalized or resolved while a chain item is mid-resolution (Rule 158.3).

So the order is: Moonfall moves Azir to the chosen battlefield (that arrival is what makes him the Attacker), then Moonfall's −2 Might applies to enemy units there — Azir included. Only after all of that does his attack trigger finalize and resolve, moving your token units in; they arrive after the −2 has already applied, so the tokens themselves are not shrunk.

Two caveats worth knowing. If the battlefield you're dragging Azir into already has a combat/showdown in progress, he arrives as a Defender and no attack trigger fires at all. And you can't play Moonfall in response to Azir's attack trigger once he's attacked on his own — Moonfall is [Action], not [Reaction], and the trigger opening the showdown chain puts the game in a Closed State where only Reactions are legal (Rule 358.4).