Locking folders in an Imac

multi user operating systems such as macOS and windows are based on if there being more than 1 user they have separate accounts on the computer keeping files and rights separate between users

but you should be able to

https://pdf.wondershare.com/document-security/password-protect-folder-mac.html?gclid=Cj0KCQiAyp7yBRCwARIsABfQsnT6yiTENTaCx3JXUIm_0HJJ0P2wY7JsruGSeMWqDkl0hTkEU7wBwHsaAqVJEALw_wcB

more

https://www.google.com/search?q=macos+password+on+folder&oq=macOS+password+on+&aqs=chrome.1.69i57j0l4.4047j0j7&sourceid=chrome&ie=UTF-8

you can password protect a folder:

https://www.laptopmag.com/articles/password-protect-folder-mac