Some helpful Github Docs

LilyGO TTGO-T-Display https://github.com/Xinyuan-LilyGO/TTGO-T-Display/blob/master/TTGO-T-Display.ino

This is an excellent api reference with examples for the esp32 BLE library combined with the arduino core https://github.com/espressif/arduino-esp32/tree/master/libraries/BLE

and here is a nice guide to ESP32 BLE programming with the espressif arduino core libs https://github.com/nkolban/esp32-snippets/blob/master/Documentation/BLE%20C%2B%2B%20Guide.pdf