When i bought an old motorcycle, genuinely all I needed was a Clymer repair guide. I'm shocked these things don't exist for more things, eg cameras.
Cameras are smaller, requiring more dexterity and often specialized small tools. And you usually have to disassemble a significant portion of the camera to repair things. And they don't (usually) require frequent periodic maintenance like a motorcycle or car (oil changes, chain lube/tension, tires, brakes, etc) - usually just repairs when they fail.
Nikon at least actually have service manuals for all their recent stuff online now https://ssr.downloadcenter.nikonimglib.com/en_US/products/29...
Think the modern stuff is easier to repair to be honest.