Q: Finder shows tags, but tags do not show any tagged files (El Capitan on Mini late 12). Any suggestions?

Sounds like you need to rebuild Spotlight, as per > Rebuild the Spotlight index on your Mac - Apple Support

(note: indexing Spotlight will take some time to complete)

The instructions say to drag a folder or an entire volume (your hard drive) to the list, not a tag.

You have to add + and then remove - the entire Macintosh HD volume in the Privacy tab of the Spotlight Preference.

Then to check the indexing, you need to click the Spotlight magnifying glass up on your menu bar and type a letter or two.

It will only re-index if you (add +) and then (remove -) it from the Spotlight Privacy box.

It is indexing when you see a blue bar.

It is done when the blue bar is gone.

The time will very anywhere from 10 minutes to over an hour depending on the type of drive and how much data there is to index.