It's a combination of a few things: existing GBS files, mapping new obscure carts, re-mapping music from revision to revision (usually they fix bugs but don't change music so it's the same overall shape, and lastly I have a crowdsourcing tool that works in Chrome if you plug in your Retro Song Bird. It allows you to play the ROM in a monitored emulator looking for register writes that look like music. Basically it's a multi-pronged approach with tons of little tools and heuristics!

I also go through and measure the music duration since some tracks are made to loop indefinitely and I measure the BPM of the tracks so you can sync two Retro Song Birds and change the tempo over MIDI and beat match them. Basically like a DJ set with carts instead of vinyl.