BLE with ESP32 tutorial part 3: many characteristics and Server Callback for Client commands

In part 3 of this tutorial we will cover two important features for the ESP32 server, namely:
1) How to use many characteristics
2) How to create a callback function so that the client can send commands to the server

Starting points for coding along:
https://github.com/mo-thunderz/Esp32BlePart1
https://github.com/mo-thunderz/Esp32BlePart2

Final code for callback function:
https://github.com/mo-thunderz/Esp32BleServerCallback

Final code for many characteristics:
https://github.com/mo-thunderz/Esp32BleManyChars

Create a mobile app for your phone to control an ESP32:
https://youtu.be/RvbWl8rZOoQ