How to study the work of a computer from the inside?

Practically – I started from scratch in printer repair. I was given to repair the killed printers sitting on the telephone line, the modem sitting on the logic board did not fail as often as the rest components.
First of all, I made a voltage table in the connectors in the chain: power supply – logic board, power supply – driver board for a working printer.
For example, the voltage for proper operation should be 5 volts for the logic board, and 12 volts for the drive board.