welcome to this wonderful community! After saving your pedalboard you just unplug the power chord, the Duo is designed like that. In fact, there was even a discussion about a dedicated option to shut down the unit but this is simply not necessary.
As for the USB: I just leave it connected all the time. For me it does not interfere with Ethernet, WiFi nor VPNs so I just don’t touch the USB cable. The Duo is recognised as network device so once you 're done, and don’t need it anymore just pull the cable.