There are institutes scattered around the world — “colleges” and “universities” — where people spend a lot of time thinking about what set of knowledge should be considered part of the baseline understanding of a subject. These are published as “syllabi”, “core courses”, “majors”, etc. I recommend finding a college or university you respect, a major that interests you (maybe “computer science” or “software engineering”), looking at the core courses for that major, reading each syllabus and coming up with a strategy to learn the material in it. The colleges and universities even offer help with that last bit!