Sometimes there's next to no difference... sometimes the quality of the components used is massive.
Without opening it up, it's pot luck really but for what the decent ones cost, the backing from the company and a warranty I would personally always buy a decent one (and did from Nomad).
I bought an off brand one on Amazon that had good reviews. I've used it for a while and it worked mostly fine. I made the mistake of keeping it plugged in to the outlet without my iPhone attached. It got very hot, to the point where I couldn't hold on to it without risking burning myself. Immediately stopped using it and went back to my Apple MagSafe battery or MagSafe duo.
Got this one from Torras 3-in-1 station for about $80 on amazon and it works just as good as my $150 Belkin one. I like this one for portability (it folds) as well as the ability to remove the watch puck and use it separately if need be. The charger has a fan for the phone which is quiet and haven't had any issues with noise. This is my new office/travel charger and i highly recommend it, especially during frequent sales through amazon. My 15 Pro in landscape fits with my S8 AW attached to it as well.
Oh and it COMES with a wall charger & usb-c cable included.
Cheap chargers warm up your iPhone due to lack of strong alignment magnets. In short, they heat up your phone more than necessary and decrease battery health.