The problem is that the airplane is a unit. Everything in it has something to do with the rest of it. Yes, there are mechanics that specialize in engines or fabric or airframe structures, but the typical A&P has to know something about everything, much like the MD who is a general practitioner, the one that sees the patient first and sorts out 98% of the problems, referring the tougher stuff to the specialists. We need a lot of GPs, not so many specialists.
The Canadian AME is the equivalent of the US A&P-IA. Here's the stuff we have to be ready to answer on the exams, and we don't have question-and-answer books like the FAA issues. We actually have to be able to cover this stuff.
https://tc.canada.ca/en/aviation/pu...ference-guide-m-rating-technical-examinations