I would like to get a wireless charger where I can charge my iPhone Xs and Apple Watch at the same time.
Is it absolutely necessary that they must be certified by Apple. In other words, how safe is it if I, for example, Amazon or another retailer. Is there anything I need to pay special attention to here like watts? I do not want to destroy my Apple devices.
Apple uses their own charging standard, as well as Qi - theirs is a superset of Qi, I believe. The newer chargers also often incorporate MagSafe, although I think that only covers iPhone 12, so is an “investment for the future”, in your case.
So a standard Qi charger will do the job, but the Apple certified stuff will possibly support higher wattage (Qi 7.5, Apple 15W, I believe) and charge quicker. Most in-car chargers, for example, are standard Qi and they will also charge iPhones.
So the extra cost of an Apple certified device is mainly for quicker charging and things like MagSafe for holding compatible iPhones (12) in place, whilst it charges, instead of relying on the stand holding it in the correct position or placing it at the correct position on a Qi charging mat.
If you can live with “standard” Qi charging times, the Apple non-certified stuff should be fine (as long as it is Qi certified and not a cheap no-name knock-off that doesn’t care about following standards), so reputable names, like Anker, should be fine.