The schematic wasn't in the C64 User Guide delivered with the machine, but it was in an additional Commodore publication "Commodore 64 Programmer's Reference Guide". The PRG was essential if you wanted to do anything more than the most basic BASIC contained in the UG.
See p.514- for the schematic https://archive.org/details/c64-programmer-ref/page/n513/mod...
Even then despite all the amazing ressource available, there is a risk of digital divide between people having access to tutor that can show the good ressource (because there is a lot of bad ressource!) and isolated people that are stuck with glorified engagement platform.
[1] https://www.salon.com/2006/09/14/basic_2/