Mechanics have been measuring voltage and following flow of electricity as part of troubleshooting process for over 120-years now. This is tried and true procedure, it definitely works regardless if you've got Model-T or Ferrari FXX. Don't skip steps or else you'll be giving us false data and we'll have to go around in circles repeatedly over and over again and nothing changes.
Before doing anything do this:
measure voltage at starter-solenoid B-terminal. Volts = ??? (DO NOT PROCEED UNTIL YOU'VE POSTED THIS NUMBER)
STOP!!! Do not pass GO! Do not collect $200 until you've posted voltage measurement above!