How it comes that some Files/Tags are not filled in completly?
Either the titles* or genre* (for example) are partly or complete missing.
And how to update/complete the list?
That depends on your used source.
How did you fill your tags?
Manually?
Using which Websource Script (like Discogs or MusicBrainz)?
With an external tool?
If you show us a screenshot of one of your "not completly filled files" and where you see TITLE or GENRE field content missing, this would help.
As Mp3tag is not involved in your list, you'll have to check with Bandcamp or YouTubeMusic or Mediamonkey or yourself why you're not getting full metadata.
Load a complete ALBUM into Mp3tag.
Use a Websource Script like "Discogs" or "MusicBrainz".
Manually select the best matching release = ALBUM.
Choose the tag fields you want to fill into your tracks.
Detailed instructions for Tag sources:
Please note:
The import works on a per-album basis and is usually a 3-step process. Please note that the search works on a per-album basis and is not automatic. Mp3tag doesn’t auto-magically detect unnamed files and tags them correctly.