I want Outlook to return messages to their folder after reading, instead of keeping them open as tabs. It used to be what happened. How can I restore it?

I'm not sure I understand your question.

Do you mean "after reply or forward"? There is an option for that.
Tips, Tricks & Other Helpful Hints: Auto-close a Message after Replying/Forwarding | Announce | University of Nebraska-Lincoln

If you do mean just viewing/reading - If you press Escape, or click the Window Close "X", the window doesn't close?

If this doesn't apply to your environment, Please see this Microsoft Support article and reply with what platform (Windows, Mac, Android, iOS) and version of Outlook you are using.

What version of Outlook do I have? - Microsoft Support

Or if you access Outlook On the Web (Sign in to your Microsoft account) via a browser, what platform (Windows, Mac, Android, iOS) and browser (Chrome, Edge. Safari, Firefox) you are using.