https://www.silabs.com/support/training/getting-started-with-bluetooth/bluetooth-le-fundamentals Nice PDF content to read and learn about BLE
how to certify your bluetooth product
https://www.youtube.com/watch?v=rrOmPjNUpYY&list=PL5CNda3uAmWxL7eqlf5MO88SjvKNKlnWz&index=5
https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/
BLE (Bluetooth Low Energy) Bluetooth 4 Bluetooth 5 Bluetooth 6 Bluetooth AoA vs AoD Bluetooth AoA vs. RSSI Beacon Technology
There are examples of decentralized messaging apps like Jack Dorsey bitchat, using Bluetooth mesh network.