Tags in macOS

The Finder tags configured on the Mac are synchronized with iOS and iPadOS 16 if you configure it. If using Pages, open the document, and then select the Sharing icon. If you select Save to Files… you will see a Tags button in the lower right of the screen, which when selected, allows you to add one or more tags to the file before saving.

If using the Files app, you would also select a file and when it opens, there will be a Share button in the lower left corner, and on its menu is the Add Tags menu item. You can also press and hold on a file icon when viewed with the Files app and the pop-up menu will have a Tags menu item on it.

Not clear, are we talking about iOS on phones, or MacOS on computers?

Well, I do not have an iPhone, so I guess I cannot help.

In MacOS if say a file or folder has a Red tag, right click on the file or folder & choose the Red flag again that will undo the flag.