I’ve been looking into Briar messenger app but it doesn’t seem to have much documentation.
Being a decentralized app, they say that communication between devices can be done over a wifi lan, bluetooth or over Tor/Internet.
But I still don’t understand how devices find each other.
This doesnt really answer your question, but years ago I wrote an instant messenger app that works over Bluetooth. Your post just reminded me of that. I also wrote my bachelor thesis about that project (linked in the readme).
https://github.com/Nutomic/ensichat