I'm wondering that myself.
I'm sure I read the details here, but don't recall exactly, how it is done. It was a bit of work that involved playing each instrument alone in the game, while recording the audio from a digital out, or similar. Sounds a bit tedious and I don't know how the tracks could be aligned doing each separately like that.
I don't know if this is done but it makes sense if the data can be extracted from the game disc itself. For DLC it would be harder from a console, unless it was hacked to get access to internal files. The stems would be archived together. Maybe easier from a PC version. But likely all encrypted.
I've seen some stems with LQ mp3 files and a midi containing a tempo map but most uploads don't. Which lends support to the idea that the song archive can be located and extracted from. Years ago they figured out how to decrypt the big Starcraft archive used to store data, and it looks fairly advanced, at least for the time. I expect RBN songs to be the same, just more advanced, but surely reachable.