How can I prevent my Library and Playlists from interfering with each other in Apple Music?
I am also a little confused but I think this information is correct:


A song that is in one of YOUR playlists must also be in your library
Therefore...if you add a song to a playlist, it will be added to your library
Therefore...if you remove a song from your library, it will be removed from any of your playlists that it was included in.
However...if  you delete a playlist it does NOT delete the songs from your library


Also note that adding Apple Music playlists (i.e. not created by you) do NOT add songs to  your library.

In my experience, this leads to my Library becoming unusable because it is a very long list of artists that only have 1 or 2 songs.  I'm never sure if I can delete the songs from my library because I don't know whether the song is part of a playlist that I want to keep.