Smoke sensor is handmade.
2 pins are used. The rest are extras.
Plug the harness in.
WHITE= CONNECT TO WHITE
BLACK= CONNECT TO BLACK/HOT
All Nest Protects have 6 battery slots and a wiring plug. In a hardwired situation the back cover has a hole for that connector. It also has a little cover that blocks 3 of the battery bays. Battery operated alarms have a back cover that blocks the port.