Double-click the green flag. Brief explanation of how it works: Each song is a costume of pixels that represent notes (x axis is pitch, y axis is time). Another sprite scans the song sprite to encode the costume into a list of notes. That same sprite then goes through the list over time, creating clones with a 'Pitch' variable that tells clones where to fall and what note to make.
Full credit to Hobson, I was snooping around the project and I found this. However I added the gif. Thanks to @freecoder11 for fixing it