So this is kind of bad advice but it's what I did to learn schematics and soldering. I bought a iPhone 6 from ebay that was screen cracked bent case (though not so bent the board was in real danger). I bought that phone for $112. I then got the phone and took the board apart and desoldered everything on it as if I was harvesting the board for part sales. I made sure not to damage any components in the removal process. I then sat down and looked at an iPhone 6 schematic from zxw tool and literally put it back together again. It took me about three months to put it back together again because some chips like cumulus were burnt from my shitty heat gun at the time. I finally put the thing back together and it powered it on and while it was not good enough repaired I would go and sell it to anyone with confidence it worked.

I found you just need to learn by doing. Buy a junker phone and rip it apart down to the bare board and try to put it back together again. Even if you destroy every single component on that board you just learned how not to do it which is as important if not more important than how to do it right.