I worked doing engine repairs not knowing much, then worked at a gas station that did repairs and learned from a master mechanic, then went to college which was SO MUCH FUN! learning the details? its so much better to cover stuff in college than have a broken motor and not know what to do first. I did engine rebuilding, transmissions, suspension, fuel systems/injection, electrical systems, brakes. I find doing repairs by the book? Helps you learn and apply skills when you dont have the book handy. With gas engines you learn how/why combustion works, Id overhaul carb's and enjoy it; Diesel? fewer people know how it works, the secret is that the compression is so extreme in a diesel and in science Pressure is equal to Temperature. Increase one you increase the other. So high pressure makes heat that lights or burns the diesel gas. Then its about pumps and getting fuel under high pressure . with Diesel often you work with very heavy engine parts and must use a hoist or lift with big tools. Those who know gas engines dont always know how Diesel works and wont touch them so there will always be work.
I made the mistake of not caring about my hands. Covered in grease/grime? Id say Im no sissy, Im tough, I will use gasoline to clean my hands. NO! Please never do that. Skin ruined to this day. Always use gloves, always clean the parts you plan to work on first, always cover your eyes with safety glasses.
If you like technology, there are engine control units that you can talk to with a tester box (mastertech ?) There will be sensors that fail and learning that part is easy (in my opinion); but just always use caution, breaking stuck bolts? there are special tools, and wd-40 is to penetrate and break down whats there otherwise you push the wrench, it falls off and you cut your hand. That harms your efficiency which means less money earned. Always safety first, always guard your hands, always have a 2nd person double check if you lift a vehicle, always use a torque wrench. do all that and you should have fun and earn good money