For this reason, each HV plug closes a small circuit in its HV socket, which is constantly checked by the car. You have to imagine it like a continuity test of this circuit with a multimeter. All the plugs are then connected in series and if one is not plugged in, the circuit is open and the diagnostic device reports that the interlock circuit is open.
The service disconnect builds on this. It's basically a red plug doing the same thing as all the others. Except that it is exclusively responsible for the interlock circuit. If service work is carried out on the vehicle, the service disconnect is removed. This means that the interlock circuit is open and no matter what ideas the car has, the HV system will not start. This ensures the safety of those working on the vehicle and it is up to you whether the HV system can be activated.
However, the service disconnect usually not only intervenes in the interlock, but also prevents the HV system from being switched on directly. Depending on the vehicle, it opens the HV circuit electrically or it opens the control wires to the HV contactors in the HV-battery. Making it impossible for them to close the HV-circuit. Even if the vehicle ignored the interlock circuit and still wanted to close the contactor.