What power supply do I need to run the Nikon Z6 II continuously on USB?

There are two type of USB chargers. Standard ones and "Fast charge" ones. The standard ones deliver around .5A (that has been the standard max output of USB ports on PCs for a while), while the fast charge ones can deliver 2.5A. These are also advertized as being able to recharge tablets (tablets have a bigger battery than phones, charging them with a .5A charger would take a whole day).

Note that the charger that comes standard with phones is usually a "slow" one, even if the phone itself boasts fast charge capability.

If you shop for one, a study by a French magazine demonstrated that many fast-charge chargers are of very low quality (voltage spikes, over-current protection, electric noise pollution (important if you use PLC for home networking), so stick with good brands (I'm very happy with several Anker ones).

It is just pretty much as simple as that. Nikon specifies that the power supply must be directly USB-C not just adapted from USB-A but pretty much any USB-C Power-Delivery will do. Many are powerful enough to power a laptop so I did not check what is the minimum required. A 30W charger is certainly sufficient and inexpensive.

To find a suitable charger, simply search for "USB-C PD Charger" and if you select with 30W or more from the results, it will be able to power a Z6 II / Z7 II. One thing to note is that many USB-C chargers have more than one port now which is great for versatility but not all ports are always PD, so be sure to plug it into one that is labelled PD on the charger or whichever is specified in the manual. I figured this out accidentally after wondering why a charger was not providing enough power one day even though it was the day before!

For illustration purposes, here is nearly the one I have. Mine is the same brand and specification but an older version. In the main photo, you can see that the top port says PD. The PD letters missing on mine but the manual indicates which port is which.