When you switch the key from OFF to ACC to IGN, you are applying Voltage (not current) to various circuits. Some circuits are always live (voltage applied) – like the hazard lights, interior lights, sidelights, headlights etc. Some are made live when the key is in ACC – sometimes radio, blower, horn, indicators, etc. Others are only mad live when the key is at IGN – starter circuit, fuel injection/engine management, ignition, sometimes radio, etc.
The voltage which has been applied to a given circuit will be present on the input side of whatever switch controls it. Once the switch is closed (switched on), then current will flow in the circuit (assuming no faults etc).
BTW, central locking is live all the time, regardless of the key in the ignition.