Since no one seems to have the same shortcut.
If you ever forget the shortcut or it's not working for you, Xcode has a built-in search to help you find it:
Make your selection (the code you want to put in comments).
In the Xcode top menu bar, go to View > Show Quick Actions.
A search box will appear. In it, type "comment".
You will see the "Comment Selection" action, along with its current shortcut. This is a great way to discover other Xcode shortcuts as well!