How do I scan for hardware changes?
Generally speaking there's nothing you have to do except launch the application that supports your hardware.

If you add a new network device, it is immediately listed the next time you open the Network preference pane in System Preferences.

If you add a new sound device, is it immediately listed in the Sound preference pane in System Preferences. (You can also Option+Click the volume icon in your menu bar within seconds of plugging your device in and see it in the list.)

You don't need to "add new hardware" to use it. If it is compatible, it generally just shows up.