He has to be hiding the monitor power cable somehow. Every Dell monitor must use a power cable and at least one video cable.
[IMAGE]On this video (February 2021) the guy has a Dell UltraSharp monitor connected to a laptop with one cable. There are no other cables visible that go to the monitor. There may be a power outlet behind the monitor, we don't see. So, is there a Dell UltraSharp monitor which can use only one cable that transfers all of these:  
Power monitor with USB