How to start with camera repair?

Mechanical film cameras are a good place to start learning camera repair because they were usually designed for hand assembly and contain simple electronic circuits if they contain any circuit at all. And mechanical systems can often be understood by looking at them closely with an experienced eye.

Modern camera electronics are often beyond repair for practical purposes because the electronic components are often complex, proprietary, or simply unavailable as working order replacements. Moreover, their workings are opaque. How an IC works can't be readily determined just by looking at it.

A modern digital camera with mechanical issues might be a reasonable candidate for repair. But it would take experience and detailed knowledge to make an informed judgement that a camera is likely to be repairable. Or that it contains parts to repair a similar camera with a different problem.

Camera repair is mostly going to be learning by doing. It helps to work alongside experienced experts, but that's probably not going to be an option. Buying some cameras and trying your skill is probably the first step.