1. Remove adware/malware 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 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
If this doesn’t help, download EtreCheck. https://etrecheck.com
Click “Free 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, select “Copy report” .
Paste the report when you reply.