Using a multimeter for your electronics projects isn't terribly complicated, but really understanding how they work takes a much deeper understanding. YouTuber Martin Lorton's three-part video guide is that exact kind of deep dive.
All told, Lorton's series clocks in at the length of 1980s comedy movie, but you'll know far more about multimeters than you'd ever thought possible. The first instalment covers safety, measuring AC and DC voltage, the basics of electricity, voltage and amps. The second part digs into how to take measurements using a multimeter with a current probe. From there, he digs into sensors, various probes and different clamps. Finally, the last part covers measuring resistance and continuity. All told, it's an extremely informative set of videos.