There’s an exchange in the actual film after they escape that’s something like:
Finn: “How did you do that?!”
Rey: “I don’t know! I’ve flown a few ships in atmosphere before, but I’ve never left the planet!”I’m sure that’s not close to verbatim but it’s definitely the gist. It’s a line I couldn’t make out until my second or third viewing because Rey and Finn are talking at the same time very fast. I took the “few ships” she refers to to mean other junkers Unkar Plutt owns, doing odd jobs for extra rations. She says the Falcon hasn’t flown in years, but I’m sure Plutt had other freighters of similar classes. I’m seeing it again on Saturday with my family, I’ll try to remember it word for word.
Doesn’t Rey talk about having worked on the Falcon for years? She does know a lot about Plutt’s modifications to the ship, and even has to tell Han how to use it with all the new stuff added. It’s possible that she’s even flown it a bit during some of Plutt’s modifications.
Rey also seems to know who Plutt originally stole the Falcon from. She’s seemingly had a pretty busy life on Jakku.