There's a lot to summarize...
MeleeAI.galaxy starts off and sends things over to the primary file for the race. Zerg.galaxy or Protoss.galaxy or whatever. From there you're sent into Zerg0.galaxy for the AI that comes with the game, or if you're using Starcrack it sends you into TriggerLibs/Zerg/ZergOpen.galaxy.
Starcrack and the Blizz AI set things up in stages, Open, Mid, Late. Poke around, let us know if you have specific questions.