Urgh, the first impression after 3 minutes is not good. First I clicked on https://www.zrythm.org/en/download.html and was greeted with a subscription model which makes me very confused. Afterwards it seems like it's only for the installer, but I don't understand why I should use a installer if it's on flatpack.
But then I install it from flatpack and start it. It says it's very intuitive. I was able to create a new MIDI instrument, a bass, and then see the keyboard and grid where I can theoretically play it. But I can't find a way to select any instrument.
After clicking around a bit I went on YouTube to find someone explaining how to do it and found https://www.youtube.com/watch?v=WoP8on5ChUQ who explains that the free version doesn't let you save anything. It's a 3 years old video but I'm thinking probably I should try if the flatpack version lets me save or not.
So I look around and see the hamburger menu top right where I hope a save option would be. I click the icon, the app freezes and then crashes. So yeah ... I guess that's what you need the payed installer for?