Notably, some vehicles like the Toyota Highlander you can’t even figure out when you need to change the spark plugs, let alone what kind of spark plugs you need from the owners manual.

Theoretically, they are to be replaced at 120k miles (!!) but they won’t even tell you that in the owners manual.

Insane.

It is, yeah. The owners manual for my current vehicle seems to mostly contain walkthroughs about the infotainment and other tech features. The section related to maintenance is tiny in comparison.

You're probably aware of this already but luckily a lot of service manuals can be found online in PDF form.

Somewhat related but I was going through an older relatives belongings not too long ago to help cleanup and found a bunch of old manuals for various household things. The detail included in them was beautiful to see - full wiring diagrams, mechanical diagrams, etc.