It changes Google to Yahoo whenever I search something

macOS Safari

   1. Remove adware 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      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.

  2. Reset search engine:    https://support.apple.com/guide/safari/customize-your-search-ibrwe75c2a3c/mac

  3. Reset  Homepage.   https://support.apple.com/guide/safari/set-your-homepage-ibrw1020/mac 

  4. Remove unknown extensions:  Safari > Preferences > Extensions

      https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac

      Download EtreCheck: https://etrecheck.com/  and post the report here.

      Click, “Free Download” button and then  “Click to download” button,

      open Downloads folder, click on it to open, and then select ”Open”.

      “Choose a problem” from the popup menu box, and then “Start EtreCheck” in the dialog.

      Click “Share Report” button in the toolbar and select “Copy report”.

      Click the “Page” button in the toolbar at the bottom of this page and paste it when you reply.

      Note: This is a diagnostic test. 

Did you try Malwarebytes as posted above?

I'm having the same problem even I did the steps stills