1. Remove any Profile you have not installed knowingly.
System Preferences > Profiles
Launch System Preferences. Do you see any profiles added to the pane at the bottom next to Accessibility?
How to remove a configuration profile? https://support.apple.com/kb/PH25680?locale=en_US
2. Use Malwarebytes for Mac to remove malware.
Download, install , open, and run it by clicking “Scan” button to remove adware.
Once done, quit Malwarebytes Anti-Malware for Mac.
Install guide: https://support.malwarebytes.com/docs/DOC-1817
Uninstall Guide: https://support.malwarebytes.com/docs/DOC-1190
The installer may ask you to allow it in System Preferences > Security & Privacy. Allow it.
3. Safari > Preferences > Extensions
Select and uninstall all extensions you have not installed knowingly.
Section: Manage extensions: https://support.apple.com/guide/safari/use-safari-extensions-sfri32508/mac
4. Choose a search engine.
Delete all text in the Smart Search field, click the magnifying glass , then choose a search engine from the list.
5. Set your Home Page. https://support.apple.com/guide/safari/set-your-homepage-ibrw1020/mac
6. Restart your Mac.
7. Launch Safari holding the Shift key down.