How do you tell if a player works best in which role?
You can check that out by yourself in the tactics screen, just change the role and see which fields are highlighted. Alternatively,
link.
How do you go about finding young players who have the potential to be amazing players?
I assume you don't wanna use the editor to check their PA. Filter for some key attributes, depending on which specific area you're loking for, as well as a low age. Or send your scouts throughout the world and hope they spot some nice players. Or use a shortlist compiled by someone else (there's a few available in the Players section).
In the case of regens, poach every single club in a country's league on their "spawning" date. Time consuming, but rewarding
Is training used to get the team to get used to a formation and work well together?
Match Prep is used to get the team to blend and get accostumed to a formation, as well as focusing on specific attacking and defensive aspects. Training is used to keep players fit and help the younger ones develop, increasing their attributes and, through your asking or tutoring, developping Preferred Player Moves.