For smaller smalle text snippets I use QR codes. I use Binary Eye on android, I'm sure there is an open source tool for Windows too. For anything larger, I use syncthing.

In that case I´d need webspace or a home server, right? Up unto now I never had a real need for any of those things, so working my way into that matter would be more work. For anyone else in this community you may be right though.

Thank you! I got it working now with the macOS using Thunderbird, Radicale and DecSync and the Android side using DecSync CC and Simple Calendar.