The Arduino Serial object is an important feature. It is commonly used, but also very underutilised.
Most people understand that the Serial Object can be used to print stuff to the Serial monitor, but less
frequently use it to receive data and use that for command and control.
In this video series I show various techniques and tools that can be used to interact with your Arduino - including remote access over Bluetooth by simply plugging in a Bluetooth module.
Access the content via my Arduino Serial Command and Control playlist.
Also, check out some of the other content on my channel The Real All About Arduino.