These IOS devices do not have a USB port, so they will connect or pair via Bluetooth.
The following Power Point presentation show you the screens, and gives you the steps to connect first and second generation displays.
As you could see in the slide presentation, the first and second generation displays would, toward the end of the process, ask you to enter a 4 digit code (1 2 3 4) before connecting. The third generation display introduces 2 different modes to connect, secure and unsecure.
If using the unsecure mode to connect, your device and display will connect without the need to enter a code.
If using secure mode to connect, you will be asked to confirm that a number showing in your display and the one showing in your device match.