Where is the tag button on iPhone?

Open the Files app in iOS. Navigate to the files you want to tag, then tap the “Select” button in the corner of the Files app. Now tap to select each file you want to tag. Tap the Sharing icon and then tag the files as desired, choosing “Done” when finished.

For More Information Please Refer: https://osxdaily.com/2018/02/28/how-tag-files-ios/