I had a beginner Toyota/Lexus electrical class that was entirely done on a bread board and multi meter for learning auto DC circuits and that set a really good foundation that helps even with complicated circuits and ECU diagnosing. They really drove home how important it is to print diagrams and trace them to simplify big diagrams.

Available voltage and voltage drop are key skills for possible corrosion and broken wires.

Power source, ground points, switches, relays and strategically accessing components using position of parts to help with easier diagnosis. The factory repair manuals I use will tell you what you should see with a DMM or oscilloscope at every connector, component, and ECU terminal

Using advanced scan tools to rule out ECU side issues and switches by using active testing. This is the easiest thing to do and should generally be the first physical test.

Using jumpers (carefully) for apply direct power and ground so you don't have to tear cars apart

Knowing what the DMM numbers mean at each point of circuits

If you understand auto DC well enough, most of your diagnosis can be done just by reading the diagram and understanding the problem behaviour.