Dear @martijn_vd_brakel and son ( 
), the annexed .aia works for me with an ESP32 and a Xiaomi phone (Android 9).
It operates on classic BT.
The app has 3 buttons and sends accordingly 'A' 'B' 'C' to the ESP via BT.
The .ino file is the ESP32 counterpart.
The ESP shows on the Serial Monitor of PC the received character and sends it back to the app (closed loop). The app shows the feedback character received.
To make it working, first of all you have to "pair" your phone to the ESP32 (in phone's BT settings).
In my app's code the IP address of my ESP32 is fixed as a text constant, you shall substitute this address with the one of your ESP. If you dont know it, you can download the Serial Bluetooth Terminal app from google playstore (free).
Therein, by selecting the menu "devices", you have to find ESP32_BT in the shown list and annotate the address, that you have to copy into the app code instead of my address.
Then recompile the app's code, download the apk to your phone and launch it.
Of course your ESP shall still be connected to your PC with the option Serial Monitor active (baudrate 115200).
That's all !
BT_to_ESP32.aia (24.9 KB)
BT_Send_Monitor.ino (1.8 KB)
Note: whether your Android is (most probably) a newer version, i.e.>= 12, you shall ask for permissions into the initialization event. You can get info about this requirement here:
or here:
About ESP32 and servo, you might have a look at:
Credits: @Taifun for his "TaifunTools" extension used to maintain active the screen and don't allow it to go to sleep, until you exit the app.
Best wishes for your project.