Plex isn’t for you or what you want to achieve. It’s as simple as that.
Because you are using MP3’s, the metadata will be defining what Plex sees. The folder system needs to follow what is outlined in the support pages for the Music Library.
Music → Artists → Albums
Plex won’t pickup folders as playlists like how you’re thinking it should.
The only “workaround” I can add to D_eight6’s point is creating a separate library for each of your folders (pointing each library to 1 of those independent folders).
Still… Plex will try to manage each of those libraries using available metadata (assuming your files are all added into those 100 folders without further structure).
Plex does let you view your music library by folders. At least, on Plex for Windows, every client seems a little different.
But sadly they do not provide any options at the folder level like “Play all in folder” or “Create playlist from folder.”
Once you dig down into a single folder to see the files, you can click the file at the top of the list and then shift-click at the bottom to select all the files, at which point you could make a new playlist. So OP can do what he wants, it will just take creating 100 playlists, ick.
A few small changes would make folder view a lot more useful.
Plex’ default behavior is to match the media and collect related metadata (e.g. from musicbrainz.org). You can configure your library to optionally prefer embedded metadata.
Plex will then organize the media by that metadata.
For music libraries, you can organize your library by artists, albums and tracks. You can create playlists / collections. Hence the comment above that Plex is metadata driven, not solely by folder dumps.
If the files have good metadata AND good folder structure, Plex lets you sort by artist, album, track, OR folder.
It is best to have the right folder structure and metadata, the bare minimum is just good folder structure.
Unfortunately, the way you describe your collection is not well suited to Plex.
If your files have at least some metadata you can probably whip them into shape with a tool like mp3tag. But, that also means changing your habits of how you use the files… that “make a folder into a playlist” thing you like to do just ain’t how Plex likes to roll.