What Chris writes is correct. As a small addition: Songs purchased in the iTunes Store with the Apple ID used in the HomePod (or one from the "family" when using family sharing) can be called up directly from the HomePod via voice command. Music that has been digitised by the user and then added to the music library can only be streamed from a device (iPhone, iPad, Mac, PC) to the HomePod - the HomePod cannot retrieve it from the Internet.