TV Episodes are being added all over the place, in wrong series... or not added at all

Plex doesn’t use ´.nfo` files.

That’s a mistake, particularly if you’re having issues with matching.

Which folder is added to your library? I hope it is a kind of top level ‘Series’ folder, which then holds the folders for the different tv shows.

You can add several folders to the tv show library. Just make sure not to spread out the episodes of one tv show over several folders. Keep them together.

Here is the quasi-official guide:

  1. Look up your show on TheTVDB
  2. Take the title as it is used on TheTVDB. If there is something in parentheses, take that too. If the title contains invalid characters (depends on your file system), like § : % *, simply leave them out.
  3. Check the naming and the folder structure of your show. Name the top folder of the show exactly after the title on TheTVDB. No abbreviations. No additional subfolders. No aliases. Add the (year) in parentheses when episode 1 of season 1 of this show was aired.
  4. Name the episode files according to the above linked naming guide. Use the same title as for the top folder. Show's Title - s01e01 - additional info.ext (‘- additional info’ can be left out)
  5. After you corrected the naming and folder structure, perform the Plex Dance with all files for this show.