Haynes publishes a lot of manuals on issues  from emissions to ac servicing.  I'd suggest looking into what's available from them.

I became interested in auto repair when I was around Waylon's age. I became interested in car electrical systems. Of course that was around 1972 and the electrical systems were pretty simple.

But through the years I've kept up on the changes.

How an engine works is pretty simple. Air/fuel, Spark, Compression.

But today's vehicles use dozens of sensors, control modules, and some networks on which the computer modules communicate with each other.

If I were young and just starting out today, I'd want to learn about those . Good mechanics are diagnosticians. They don't just see a code and replace the part the code is referencing. Most mechanics today can't even use a scope. 

So I'll preface this by saying I have no affiliation. I don't belong to his premium channel and I didn't buy his book. I've already learned that stuff.

But I like his teaching material. (he has a free youtube channel. https://www.youtube.com/c/ScannerDanner/videos )

You can get an idea of what he does watching the free videos.

Paul Danner is a teacher at Rosedale Technical College. He teaches electronics and electrical systems on cars.

He has a premium channel for something like $11/month and a textbook that goes along with it. (the printed book is pricey, around $100, The ebook is around 1/2 of that)

The premium subscription is recordings of his technical college classroom lessons with his students. He teaches, they ask questions, and Waylon would be sitting in on an automotive  technical college classroom for $11/month. 

That's a cheap education.

Maybe look into that if the free videos appeal to Waylon.

https://www.scannerdanner.com/join-scannerdanner-premium.html