You download the repo, learn the stack, look over the codebase, set up the environment (someone might have to help you on this at first with a screen share call if there's no documentation), work on some small tickets, ask questions, and get feedback. Easy. I've always gotten up to speed and productive within a week or two this way, without there being any real onboarding documentation or plan in place to boot. Never had a serious job in an office but I feel like it would have only slowed me down.