https://techcrunch.com/2017/09/25/apple-switches-from-bing-to-google-for-siri-web-search-results-on-ios-and-spotlight-on-mac/

Apple is switching the default provider of its web searches from Siri, Search inside iOS (formerly called Spotlight) and Spotlight in Safari on the Mac. So, for instance, if Siri falls back to a web search on iOS when you ask it a question, you’re now going to get Google results instead of Bing.

Consistency is Apple’s main motivation given for switching the results from Microsoft’s Bing to Google in these cases. Safari on Mac and iOS already currently use Google search as the default provider, thanks to a deal worth billions to Google over the last decade. This change will now mirror those results when Siri, the iOS Search bar or Spotlight is used.