Each category searches within a curated set of websites.
Try "jupiter" or "shoe" in all categories to get a feel.
There are unfortunately high chances of duplicate images of different sizes being returned.
BetterRef is created by Gerben Pasjes to help artists and designers find better reference, which has gotten increasingly difficult since AI image generation.
Additional help: Robin Zaagsma, Jessica Piatti, Steff Kempink.