Hi thank you! Sorry for the delay in replying, I'm sorry but I'm not sure I 100% understand what you mean? The way I map the music is that I first make the music in a DAW (garageband in this case) then I export the audio. For the note positions, I look at the tracks in garageband and write the note positions all out on paper first. I then add the exported audio and the note positions in the Unity inspector. I'm not sure if that makes sense or it's a bit too specific? The simplified answer is that because I make the music I can just write down the note positions as a number and enter those into the game engine along with the music. Let me know if you have any more questions!
If you want to make your own music you can use the beatmaker mode to track beats, you can also edit the saved beat text (the blocks of text people use to share beats) directly if you prefer. Here's the thread on sharing and loading beats: https://itch.io/t/1690108/share-and-play-beats