A basic Unity script interface that opens a serial port for communicating with an Arduino that is running a sketch similar to this
https://create.arduino.cc/editor/DsharrockACM/0ec242b3-be66-4bff-a106-e55195c35703/preview
Contents:
ArduinoInput.cs - The main script that monitors and reads lines from a serial port and returns strings on request via public string GetInput()
SampleScene.unity - A basic scene with a GameObject with an ArduinoInput component on it
DebugWriter.cs - Awaits a press of the spacebar then calls the GetInput() method and writes the result to the console
LeftAndRightMovement.cs - An example of how to convert short string outputs from Arduino into basic object movement. Polls GetInput() continuously so denies any input to DebugWriter.