Tutorial : https://www.instructables.com/id/Arduino-Robot-Car-Bluetooth-Controlled-and-Program/

This app is designed to make it easy for you to build a Basic Arduino robot car that is equipped with circuit diagram and code. The app allows you to control an Arduino based robot car over Bluetooth, and also you can upload Arduino sketch/code directly from your Android phone to Arduino via USB OTG or Bluetooth.

Features:
- Simply remote control interface
- Upload firmware via USB OTG directly from Android phone
- Remote control mode
- Obstacle avoidance with servo mode
- Line follower mode
- Object follower mode
- Available Arduino source code