Launch Safari holding the shift key down.
Follow steps given here: About pop-up ads and windows in Safari
Remove adware if installed on your Mac.
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