They have their own index on top of using Google's. As such they do some of their own ranking like promoting the "small web" and surfacing more Internet blogs, for example. You can also customize results rankings by domain -- for example, when I search for an image I've personalized it to block social media results, Pinterest, and AI-generated images (they tag AI images and they're reasonably good at it).

The end really is that I can have confidence that my results will be relevant from the first result -- no sponsored content, no ads, no unwanted AI slop (you need to purposely invoke AI summaries, for instance, by ending your query with a question mark), and no domains that I find give low quality results. There are even more customizations you can do and I could wax poetic about Kagi, but at the end of the day a good search engine helps you find useful information and gets out of your way, and I haven't seen a search engine do that better than Kagi yet.