Dell power adapter does not get warm?
If they don't get warm when not supplying power, then they have low quiescent power.  There are several ways to achieve this:

These supplies could be optimized to use very little power when not supplying any.  That could be well under 1 W, to the point you wouldn't notice the temperature increase in a case that size.

As you say, there could be a special circuit that disables most of the power section if nothing is attempting to draw current.  That still requires some standby DC power so that attempts to draw current from it are detected.

There could be private communication between the laptop and the supply.  The supply doesn't turn on fully until the laptop identifies itself and requests the higher power.  This mechanism can also be used to deter competitors offering the same supply for less money, without the large markup for the brand name.



In the end you'd have to get information from Dell to know for sure.  My guess is #1, since it's really not that hard to do, and simpler than the other means.  With the USB type-C power managing standard, I expect a lot of these proprietary chargers will disappear quite soon.  Many new products already use USB type-C for charging.