The onboard regulator will accept from 1.8 to 5.5 volts. Pin 39 is the place to give it power from sources other than USB. This is "Vsys". It directly feeds the onboard regulator, and a diode prevents it from feeding power back to the USB connector.
However if the supply connected to Vsys has a voltage less than 5 volts minus the diode drop, you will get power through that diode that your supply may not be happy with. You may want a diode to protect your supply. If you are providing 5 volts on Vsys, things should be fine.
I am now running two Pico devices from one ordinary USB cable. As we all know standard USB has a 500 mA limit, so a single Pico pulls less than 250 mA of 5 volts.
One fellow measured 38 mA running some python code. Another sees 18 mA. Sparkfun says that their RP2040 based "Thing plus" board draws 24 mA on the avarage without any code running.
Another question entirely is how much 3.3 volt current you can pull from the regulator on a Pico to power other components, I see number on the order of 250 to 300 mA. And yet another thing to keep in mind is that the RP2040 has sleep modes that pull very little power.
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