alt Hacker NewsMusic is broken down into two copyrights: Masters and Publishing.
Masters is the recording themselves, made by recording artists, distributed by record labels and distributors (DistoKid, TuneCore, CD Baby, etc.)
Publishing is the "work" or the song... made by songwriters, and royalties are processed by Music Publishers and PROs (ASCAP, BMI, SESAC).
A good way to wrap your head around it is... I can record a cover of Katy Perry's Firework... I can distribute that master (and make money) to DSPs like Spotify etc... but I do not own the "song" Firework.
The Mechanical Licensing Collective (the MLC) is the body that governs US digital streaming mechanical royalties, which is a right on the publishing side.
Just a note ... "mechanical" royalties are publishing royalties payable when a duplicate is created. Back when the duplicate was sheet music or a record pressing that made sense linguistically. Nowadays, mechanicals are due when a stream is done or a digital copy is made.
Public performances like a live concert get a different category of license.