For me, it's kind of a mixture of having notes scattered around my devices, things I spontaneously come up with in my head and key information from lots of different wikis or the series themselves, where characters & worlds appear in, since my project is a crossover-story and those play a major role in it and I wanna make readers feel like, when a character appears, meeting an old friend.
Personally, I have little experience with Google Docs besides going through occasional polls or forms, which is why everything is pretty much all over the place, but because I've been on my project since, like, always, I have almost 0 problems with keeping up with what comes after whatever event and I've gotten pretty much used to it ^^"
(And I've written the script over god knows how many times for any major change, so the storyline is basically eternally engrained in my brain lol)