You can open an upper level tag (or any tag) in a separate window and then view the tag filter for just that tag hierarchy.
As you drill down, you’ll see the tags you select and also tags that are applied in common.
For instance, a document is tagged with both “drivel” and “useless”. If you drill down to the drivel tag, you’ll see the useless tag as well. If you expand useless with the little triangle you’ll see the documents tagged as useless which are, furthermore, drivel.
In my case, too many such notes. I’m such a poor scholar.