This is minor, but I can’t help it. Before the briefing at the rebel base, is it ever established that Finn was stationed at the Starkiller?
Well he was the janitor of the Starkiller base! (Not making this up). Either the janitor or the garbage man. Maybe both.
It just seems to come out of nowhere at the end of the movie that in addition to being this hero, he also knows that the weakness to the thermal oscillator is in sector 45 or whatever.
Well, as the janitor, I’m sure he had a lot of sweeping to do all over Starkiller base. So I’m sure he got to see lots of stuff and overhear many conversations.