Why my safari always redirect from google to yahoo by itself?

Adware infection?

Go step by step and test.

   1. Remove unknown profiles if present.

      System Preferences > Profiles

      Open System Preferences,  select “General and click  the “Profiles” icon ( a checkmark on a gear) .

      When Profiles pane opens, select the unknown profile and click the minus button at the bottom.

      Section: Remove a configuration profile from your Mac

      Profiles: Use configuration profiles to standardize settings on Mac computers - Apple Support

  2. Remove unknown extensions:  Safari > Preferences > Extensions

      Get extensions to customize Safari on Mac - Apple Support

      Change Extensions preferences in Safari on Mac - Apple Support

   3. Check for adware.

       Run the latest release of Malwarebytes for Mac to remove malware/adware, if installed on your Mac.

       For instructions: Install Malwarebytes for Mac v4      Uninstall Malwarebytes for Mac

       Click the “FREE DOWNLOAD” button.

       Click the “Scan” button. Once done, quit Malwarebytes for Mac.

       Restart the computer and relaunch Safari holding the shift key down.

       Scan for Malware again.

       Additional steps  if necessary.

  4. Reset search engine:    Customize searching in Safari on Mac - Apple Support

  5. Reset  Homepage.   Change your homepage in Safari on Mac - Apple Support 

  6. Restart the Mac. Relaunch Safari holding the Shift key down.