Explain non-nicotine vape/hookah pens to me.

I think you might be a bit out of the loop, so to speak.

You can buy a full kit EVOD clone online for less than ten dollars, this includes a battery, USB charger, and clearomizer. Some even come with some nicotine liquid, or you have to buy it separately. That is all you need to start vaping, there are THC or research chemical analogues of THC available but they are not sold openly except on the darknet TOR markets(think Silkroad clones).

I don’t use them myself(don’t care for THC) but they are available but rare, most likely whoever you are talking about is just using nicotine.

EDIT: Disposable electronic cigarettes cannot be refilled, period. They simply cannot it is a sealed system that can’t be accessed to fill with your drug of choice, but on the flip side I’ve never seen one with 0 nicotine.