Well, you wouldn't be the first guy to learn the trade toally by the seat of your pants. But, honestly, it has gotten more complicated. It 50 years ago, you had cast iron drains, copper or galvanized water pipes. That was about it. Today, there is PCV, CPVC, PE,PB,PEX, APS. Then for gas there is epoxy coat, CSST, and the new plastic GasTites. There are tankless WH, condensing furnaces, radiant heat, and on and on. Codes have become even more detailed.
SO, I would suggest that there is an unavoidable need for some "book larnin'". Try to find a trade school or union apprentice program that you can get in. Possilby your employer will support this, possbilby you will have to do it nights and weekends.
I just finished reading the June issue of the IAPMO magazine OFFICIAL.
It is very clear that "green" is the wave of the future in the plumbing arena. There will be a flood of new products, technologies, and codes. The need for well educated and well skilled tradesman will continue to grow. This should be a lucrative career with unlimited opportunities for someone young like yourself. Be hungry for knowledge!