How can I learn smartphone repair online?

YouTube is the key resource.

I bought an electronics repair kit from Amazon that’s got a bit for every screw imaginable. They’re like 20 bucks.

I still use my iPod video out of nostalgia, I have old iPhones and tablets which I use at home to run various tasks (they work surprisingly well).

Messing with iShit can be very exhausting because everything is locked, you are got to have some really expencive programmators from your local wizards to move towards repairing those.

I am amused of why are you opening the thread with expressing your love to smaller phones, not your love to soldering under microscope and messing with BGAs and repairing all neighbours' Nokias after your school classes, so I am almost sure you are a wrong person to do this kind of job no matter how hard you will try.

Don't afraid of leaving your phone into hands of professionals, they typically have too grinding schedule to read your iCloud files or harm you using the hardware vector.