It would be helpful if there would be an option to select multiple sources on different drives and unify them as a single source in Plex media server so every tv shows will be shown in the same playlist even if they are sorted in different drives on the same computer.
You can do this now. Any one library can have multiple folders from any drive that the server has access to…
Read it again. The requested idea is to UNIFY the folders. Not to add more folders.
Look at that, this is my Library:
link
A, C, Cartoni and M are all Tv Shows and I want to make it look like a single folder instead of 4.
Right. You’ve created three TV Libraries each with one folder in them.
Instead, you should create one TV Library and add all three folders to it. Then it will be unified.
Click on the … on the right of any library and select “Edit”. Then on the left side, select “Add folders”. Add as many folders as you like. Then click on Save Changes.
Thank you I got it now 
I can’t believe I’ve been trying to do this for years now… I feel so dumb 
Sometimes the simplest of things…
Glad you’ve got what you need!
Early 2021 clean-up: implemented