From the starter switch.
As you guess, the big terminals take the high current supply from the battery and switch it to the starter motor.
The solenoid has to "know" when to do that. There will be a wire from the ignition switch that is energised when the switch is in the "Start" position.
The negative connection to complete the circuit is simply via the body of thexsolenoid, once it is bolted in place.
Krrhf, April 2025
