How do you tag a photo on a Mac?

Tag a file on the desktop or in the Finder: Select the item, then open the File menu. You can also Control-click the item, or tap it with two fingers. Choose a color below Tags, or click Tags to choose from more tags or enter a new tag.

For More Information Please Refer: https://support.apple.com/guide/mac-help/tag-files-and-folders-mchlp15236/mac