A service manual is a guide and some are better than others.
A good tech knows circuits and can repair without much manual support.
A great tech can look at a unit or circuit and know what to find and figure out voltages themselves.
When I could fix gear without a manual, I decided I might know enough to work on other people’s stuff. For the most part, I only need a manual to get factory settings on the amps.
Units with separate boards for the sub-systems are fairly easy to work on without a manual. Once you get enough knowledge and experience, you can recognize which board is what and look in the right place for the problem.
Electronics repair is a skill and an art. Not something you can learn quickly.