Scanners are responsible for getting your media into Plex. This is in contrast to Metadata Agents, which are responsible for getting posters, artwork, descriptions, ratings, etc.
Custom scanners live inside your Plex Media Server data directory in:
%LOCALAPPDATA%\Plex Media Server\Scanners\
macOS / OS X:
~/Library/Application Support/Plex Media Server/Scanners/
Scanners are Python files that live in one of three subdirectories, for movies, music, and TV shows:
Movies/<br /> Music/<br /> Series/
By default, neither the "scanners" directory nor the subdirectories will exist, so you'll want to manually create them. Once you install a new scanner by dragging the file into one of these directories, you'll have something like this:
....Scanners/Music/Plex Music Scanner (no tags).py
Then restart your Plex Media Server and you'll be able to select the scanner when you create a new library.