How do I switch back to Safari as my search engine from google?

Safari is an internet browser, not a search engine. Possibly you mean that you have Google Chrome (which is also an internet browser similar to Safari)? If so, you can simply not use Chrome and use Safari instead or if you want to, you can delete the Chrome app from your iMac. See: Uninstall Google Chrome - Computer - Google Chrome Help

Safari > Preferences > Search (at the top where you saw General).

There is also malware that will change the search to Yahoo. The following can help with that: Malwarebytes | Free Cyber Security & Anti-Malware Software

Safari Preferences/General/Default web browser.