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The water analogy is perfectly fine for many situations, but the reason these don't draw a lot of power when nothing is plugged in isn't because a "valve is off". There's a transformer, so this is like two separate water lines. If the charger is plugged in, there's always a closed circuit on the mains side of the transformer, even if there's an open circuit on the DC side. See the first diagram here: https://circuitdigest.com/electronic-circuits/ac-to-dc-converter-circuit-diagram.
The reason new chargers don't use as much power with no device attached is because of better design. If you checked an old charger or some crappy power supply, they'll use a fair bit of power even with nothing on the DC side. It's not enough that one would matter, but it is enough that there was an industry wide initiative to reduce phantom load resulting in new chargers that use almost nothing when nothing is on the DC.