This is what I found on a web search. I don’t use Safari but it still uses Google if I use Siri.
AI Overview
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Apple uses Google’s search engine for Siri primarilyfor consistency with the default search engine in its Safari browser, according to SMB Nation.This ensures a unified user experience when searching through Siri and navigating the web in Safari.Additionally, Google has a strong relationship with Apple, and the move is seen as a way to deliver the best possible user experience, according to SMB Nation.