Hmm, just plugging one of the buttonless ones I have in to a recent-ish Asus Vivobook with a tester inline, it shows 19.6V / 1.68A. The original charger for this laptop was 45W, so this is not quite as performant, but close enough for my purposes.
They are designed for laptops so I would expect them to in theory be choosing the right voltage for whatever brand the plug was designed, but they should not be used on random electriconics unless they specify the output voltage!
a) just pass through 5v from the charger
b) Support PD and have a button to let you cycle through voltages (so you have to be careful not to fry your electronics)