Bluetooth is the connection between the phone and the backup camera. It's used to send requests and commands to the backup camera.
Wi-Fi is for video and image transfer which is faster than Bluetooth.
Once the device is successfully connected to the ZUS App, the Wi-Fi will be turned on automatically and in the meanwhile, you will see the LED on the cam turns into Blue.*
*Only later models have this feature. If your device model is ZUSRBKUALZ then it should have this ability.