How to turn off 'safe search' on MacBook Air?

If you’re referring to content restrictions in Safari, that’s managed by Screen Time. You must have access to screen time to change that setting, so for example, if I enable the restriction for my kids’ accounts that I manage with a Family Sharing group then my kids cannot disable the setting.

Change Content & Privacy settings in Screen Time on Mac - Apple Support

Your concern could be interpreted several ways. If the others' replies don't address it, the following may be applicable: Removing "Search Marquis" / "Search Baron" / etc on your own - Apple Community

Safe Search is a feature of a Google account or your internet provider who may have a Safe Browsing setting that you can turn off for your account. Many times a VPN will also have a feature like that.