If we are talking about custom videos, which are neither listed on TheMovieDB.org, nor on TheTVDB.com , read on.
First off: I recommend to use the mp4/m4v container format for these videos. Because this is the only one, from where Plex can read more than basic embedded metadata.
I urge you to store as much information about your videos in the mp4 containers’ embedded metatags. This makes it less hurtful if you have to break down your Plex server or the library and recreate it.
mp3tag can be used freely.
Use at least the ‘Title’ meta tag for the title of the episode.
‘PODCASTDESC’ can be used for a description of each individual episode.
If you want to make them into a tv show, you need to create a library in Plex of the type ‘tv show’.
You should keep your self-created shows separated from “real” tv shows which you may find on TheTVDB.com .
Put them into a separate folder and create a separate library in Plex for them https://support.plex.tv/articles/categories/plex-media-server/libraries/
The files must be organized into folders and subfolders, just like a real tv show in Plex.
The file names must contain at least the season/episode number code, e.g. s01e03
If you have a person appearing in different “shows”, you could use Collections to tag these shows.
If you know how to embed ‘actor’ meta tags into mp4 files (which is unfortunately non-trivial), you could use these instead.
However, you cannot tag individual episodes with different cast or collections – only complete shows.
For non-“episodic” content, you better use a library of the type ‘Other Videos’. The file names fopr this library type must not contain the season/episode code or they will be ignored by Plex.
P.S. If you populate the ‘Album’ tag, Plex will add the video to a Collection of the same name. (although that is mainly useful for ‘Other Videos’ content)
Avoid “reserved” words in your file names, or those files won’t appear in Plex: https://support.plex.tv/articles/201381883-special-keyword-file-folder-exclusion/
If you describe some actual examples, we could advise you better on which approach to take.