The cars I work on are from the early 90s and everything is very simple to understand.
e.g. Electronics are normally simple circuits that aren't much more complicated than what you would find in a door bell and finding faults is normally just tracing wires and using a multi-meter. I had issues with the brake lights / reverse lights not working, the issue turned out that the spade like connector in the fuse box was pushed through and was making partial contact. Price to fix this was £0.
EDIT: Just remembered this isn't accurate. I had to buy a new reverse light. The entire reverse light assembly was ~£20. So the price to fix was about £20. The light assembly itself was like a big bicycle light.
My newer car needs a OB-II scanner to diagnose anything with a phone app. While this is arguably quicker it can be misleading. Sometimes it will be telling you that something is malfunctioning but it is really the sensor itself. These sensors are £200-£300 a piece. Replacing 4 glow plug sensors cost me £800. I was paying essentially to make the "you must service your engine" light to go away. There was nothing wrong with engine itself.