There's not much to it, it's just a magnetic case/shell into which you place the batteries. Xtar even allows you to use smaller batteries in it as well (half-size 18650, forgot the correct designation), can't really mess it up in any way as far as I know.

I bought it as a charger for 18650 since I have some lithium battery flashlight, but the power bank function ended up being even more useful.

Some new flashlights have a reverse-charging powerbank function too. Probably Wurkkos of Sofirn. Though don't fall too deep into that flashlight enthusiast rabbit hole. If you've never owned an enthusiast flashlight, it's amazing how powerful they are compared to the stuff they normally sell in stores. And interestingly, quality western made flashlights are very rare, the chinese are the best in this field and you can get the best flashlights easily on aliexpress.