This is just from personal memory but if i remember correctly; I like the version from "Reign of Fire" (2002) where they have 2 different chemical pouches in their mouths that mix and create fire. Kinda how snakes have a venom sack but these are two different chemicals that when combined create fire.

That author's explanation has some overlap with the explanation of dragon flight and dragon fire breathing from the 1982 animated movie "A Flight of Dragons".

In that story, and human from our work is transported to a fantasy world, and there, transformed into a dragon. Here's an elder (red) dragon (not a human transformed) explaining to the transformed human (green dragon) how to be a dragon, including flight and fire breathing.

3 minute youtube clip from the movie

Magic.