Safari has a set list of search engines, you cannot add to or remove from the list.

They currently "support" safari currently by redirecting the searches that go to your chosen search engine to kagi.com with an extension.

On iOS at least, Apple does not allow custom search engines in Safari and does not list kagi. So the kagi app redirects requests to a different one. Feels gross and dumb.

It really isn't that bad. Setup is really quick and I have not found cause to complain on mobile.

Same. It’s annoying that it’s required, but once configured it works just as I’d expect.