Simon Müller

XMPP can indeed be used for some other things

For example, Google uses it to this day for Google Cloud Messaging - the push notifications service in Android.

Just the how is something I cannot answer :/

Obsidian isn’t FOSS at all, sadly.

Though, they’re not the data-hungry kind of proprietary, or use some proprietary format. It’s basically just a really fancy markdown editor with plugin support. No telemetry, no accounts, and sync can be self-hosted.