How to return all playlists that include a specific track

[ssharkss]

Hello, I'm interested in creating a tool that returns all playlists that include a certain track. The tool would be similar to the one used in these websites: 

• https://www.chosic.com/spotify-playlist-search-tool-by-song-or-artist/
• https://vibify.net/

Except I also want to return songs that most often appear in the playlists that include the song in question. Essentially, I want to find "similar songs" to a query using user playlists.

Here's the trouble - I can't figure out how the sites above are searching "all users" or "all playlists". How is this possible with the Web API?