A practical way to start is with older mechanical film cameras, not modern DSLRs. Mechanical film cameras were typically designed for hand assembly, and their mechanisms are easier to inspect and understand. If they have electronics at all, those circuits are usually much simpler.

Modern digital cameras are much harder to repair because many faults involve proprietary electronic parts, integrated circuits, or assemblies that are difficult to diagnose and often not available as replacement parts. Even when a camera can be opened, the actual failure may not be obvious or economically repairable.

A modern digital camera with a clearly mechanical problem may be a reasonable learning project, but judging that correctly takes experience.

So if you want to learn, start with inexpensive mechanical film cameras and build skill in understanding shutters, linkages, and assemblies. That experience can help you decide later whether a newer camera has a realistic repair path. For repair shops, the useful background is likely hands-on experience with fine mechanical assemblies and camera-specific troubleshooting rather than just general interest in electronics.