Bing is the default search engine for Siri, and therefore, whatever search phrase or term you feed to Siri, s/he will bring and show the results from Bing. Now as a Google lover, you would certainly like to see results from your favourite search engine. As a matter of fact, more than 70% internet users rely on Google for any information. There are many users who are loyal to Yahoo! and therefore, they want to search queries on Yahoo!.
However, Siri's allegiance with Bing is so firm that Apple doesn't allow to make any change from the Settings app to select any other search engine aside from Bing. But you can always ask Siri to search your query in your favourite search engine, i.e. Google or Yahoo!.
Before you start following this information, make sure that you have enabled Hey Siri Hands-free mode on your iOS device.
Ask Siri to Use Google or Yahoo as Search Engine on iPhone/iPad
If you have enabled Hey Siri, you don't need to press and hold that Home button on your iPhone. Otherwise, you are supposed to press and hold that button to summon Siri.
Siri for Google
When Siri appears on the screen and asks you to feed a query, simply speak: Search Google for (search query/phrase/term).
Siri will start searching your request in Google and list down the results.
Siri for Yahoo!
Again, summon Siri and command him/her: Search Yahoo! for (search query/phrase/term).
Upon this, Siri will bring some results from Yahoo! search engine for you.
If you want to use Bing for search results, just speak: Search web for (search query/phrase/term).
You must have noticed that while searching Google and Yahoo!, Siri brings you to Safari page, which is a default browser for iOS. And for Bing results, Siri keeps them within its screen.