Engine will only start with AUX Battery Start

I am quite mechanically challenged but OP in his post said that when he turns on the ignition, that there is 'clicking' which I assume is probably a solenoid? Then follows some suggestions that perhaps the circuit might be interrupted via a battery interrupt switch. If there is a battery interrupt switch, it would seem to me that it would be located as close to the battery as possible so that zero current would go down the line to be drained by things like a radio and other power drains. If the battery is effectively disconnected that way, how would power be getting to a solenoid? Do solenoids have their own little dedicated battery?