FamiTracker is where NES music is written today. Each of these songs was exported from it as text (File, Export text) and made once into a .song with its samples, which is what the editor does when an export is dropped on it. A .song loads at once with useLoadSong; the export would have to be read every time.
From there they play on the engine's NES like any other song, and their tempo and key change while they play. Only the song chosen is loaded, and one loaded before is not loaded again.
Left and right for the game, up and down for the song, space to play, [ and ] for the tempo, - and = for the key.