A simple Serial compatible library using ESP32 classical bluetooth (SPP)
Download one bluetooth terminal app in your smartphone
For Android: https://play.google.com/store/apps/details?id=de.kai_morich.serial_bluetooth_terminal
For iOS: https://itunes.apple.com/us/app/hm10-bluetooth-serial-lite/id1030454675
Flash an example sketch to your ESP32
Scan and pair the device in your smartphone
Open the bluetooth terminal app
Enjoy