The guy who fixes your bike has a PhD in something you’ll never read about. Ask him questions.

Deep knowledge accumulates anywhere someone does the same demanding thing for years. A mechanic reading a sound, a cook judging heat by smell, a nurse noticing something off before the machine does. None of that gets written down, and it is easy to walk past because the people holding it are not labeled as experts. It is still expertise, and most of it is available to anyone who asks a decent question.

Ask the next person who fixes something for you how they knew what was wrong. Ask it like you mean it. You will get two minutes of something no course sells.