Kagi does have its own search index of niche sites, they even have an API for it. This is combined with Google results; they used to combine with Bing as well but stopped due to Microsoft charging too much money.

Very different from DuckDuckGo and Startpage which are still just a Bing proxy and a Google proxy respectively.

Not just Google, they use Bing and Yandex(?, might be something else) too, I think.