Ask HN: Advice on learning an apps file format?

I have a lot of Apple Notes. I want these notes on platforms that are non-Apple. I thought this would be a good learning experience to learn something new while scratching a personal itch. However, what ways are possible to find out a file format, like Apple notes? Use a packet sniffer and start looking at each packet when I am working with notes? I honestly am drawing a blank on how to even get started.