Safari always turns to Yahoo search

   1. Remove adware if installed on your Mac..

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

       For instructions: Install Malwarebytes for Mac v4     

       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.

       Uninstall Malwarebytes for Mac.

       https://support.malwarebytes.com/hc/en-us/articles/360039025233-Uninstall-Malwarebytes-for-Mac

       Additional steps  if necessary.

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

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

  4. Remove unknown extensions:  Safari > Preferences > Extensions

      Get extensions to customize Safari on Mac - Apple Support

Please read the following:

Removing "Search Marquis" / "Search Baron" / etc on your own - Apple Community

That User Tip describes how to examine three separate folders and capture screenshots of them. Read that User Tip, take those three screenshots, and post them in a reply to this Discussion.