MIT app Inventor is what you are looking for.
On “Learn ESP32 with Arduino” you will find the
Project #3: Bluetooth Low Energy (BLE) Android Application with MIT App Inventor – Control Outputs and Display Sensor Readings
That shows how to use.
Can you provide more details about what you want to know?
There are many different ways to control the ESP32/ESP8266 using your smartphone: local web servers, web apps, cloud web server, using third-party services, etc.