I'm a bit more optimistic about the future but I certainly share the concerns of @mayer and @danj. This happened many years before when there was thriving repair community for luxury items such as TV's, VCR's (!), Stereo's etc. Check out the ads in this [https://www.americanradiohistory.com/Archive-Radio-Electronics/80s/1985/Radio-Electronics-1985-01.pdf|mid-80’s magazine]. It's even more prominent in older magazines.
Almost nobody has those fixed any more and hardly anyone has the skills to do so. Back then, the manufacturers supplied schematics and if you were trained in electronics, you could fix just about anything.
Today, everything is a state secret, devices are designed and manufactured to be un-repairable. Apple is using less and less "off the shelf" components and designing in house and thus controlling the supply chain. That said, there is a thriving repair community for Apple devices are they are ubiquitous and hold their value very well. It's worth it to have them repaired. But as others have noted, you need to bring something to the table. The days of easy money swapping screens is gone or dying fast and you need to be able to do something others can't or won't invest in. Refurbishing, micro-soldering, data recovery...all of these are hard but will protect you for a good while.
So start simple, get a pro toolkit from iFixit, buy a few dead or damaged devices and practice on those. That will let you see what other tools and supplies you need and when you feel comfortable, try fixing a few phones for friends and family (they will be more tolerant of mistakes). Then move up and onwards but never stay still.