Need steady hands. If you have trouble painting miniature with a tooth pick you gonna find it hard to unsoder parts and replace them.

Testing phase usually people look for a bunch of same model phone and use ElE tester to determine which part not functioning then part swap to build a phone.

I last time use to play with buying cheap dead gameboy to fix. Its fun but not easy. Can totally learn everything free from the internet.

Price come when you hunt for specific items to replace instead of trying to rubble trough discarded stock for parts.

If you are only interested in specific electronics only consider going vocational school for deploma. Cause I don't think phone repair shop will take in hobbiest to train for work.