For modern vehicles, the dealers really don't use manuals anymore. They connect their proprietary diagnostic computer, it runs all the tests and figures out what is wrong, and tells the mechanic what to replace or adjust.
You can sometimes find bootlegs of these computers (generally a "tough" Windows laptop with software and connecting cables). They usually don't have the latest and greatest version of the software, but if your car is older/out of warranty it's generally covered. I don't know if they get them from a recycle stream or dealer mechanics sell their old ones or how it works.
The computer diagnostics don’t tell the technician how to do the replacement. The service manual has all of the procedures, parts lists, and special tools necessary for the job.
This doesn't make any sense. Many regular issues are not diagnosable by computer. I have legacy access to some current BMW service data and it does not behave the way you describe at all- I only have to whip out the scanner for certain electrical and computer issues, and to update service schedules.
Everything on a modern car runs through the computer. Climate control, entertainment systems, windows, lights, seats, various driver aids such as cruise control, lane-departure, blind spot warnings, hands-free driving, automatic/antilock braking, traction control, etc. as well as the engine and transmisison. Sure, something like a failing wheel bearing would still be diagnosed by the mechanic, but they need the scanners for most of the problems on a modern car.
You're right, but just to have fun with the example you gave- some wheel bearings now incorporate a wheel speed sensor. So even those might, in fact, be first diagnosed by the computer, depending on the failure mode.
You can sometimes find bootlegs of these computers (generally a "tough" Windows laptop with software and connecting cables). They usually don't have the latest and greatest version of the software, but if your car is older/out of warranty it's generally covered. I don't know if they get them from a recycle stream or dealer mechanics sell their old ones or how it works.