The android, windows and Linux app use propriety parts, the android app is a web view app built in android studio, the Linux and windows apps are just electron apps. I have all the source code I used to build them on my pc, but I didn't create a repo for them. go-notes is fully open source with the backend in go, the clients are just extremely basic frontends.
Edit
Just gone and checked and all the electron apps source code is in my repo, so the Linux and windows apps you can build yourself from the code. I can upload the android code from android studio if you want as well?