Zerodaymp3 and others get their music from ripping groups, the most famous (well-deserved) being RNS. They have shells, FTPs, and fast lines to distribute rips (as tar files) to high priority users first. Once the reaches them, they can distribute it to others, and so on until the lowest level is reached (Audiogalaxy/Blubster/FastTrack/Gnutella(yuck)/etc.) The archives are interesting, containing an nfo file (usually with nice ASCII art), an sfv file (for checksums, use 'sfv'), a playlist as m3u, and of course the MP3s. These files are usually stripped away not shared on garden variety P2P.
I discovered another use for IRC (other than chat) today: MP3s. On Zerodaymp3, they distribute latest releases. This (IRC and FTP) is where your music comes from before it reaches other P2Ps, folks. Zerodaymp3 requires no contribution to join or download, and unlike #mp3xdcc I rarely see advertisements. Of course, it's just one place to get the latest releases, but there's no junk there and the bot's connections are fast.