Data transfer from ARDUINO to PIXHAWK through serial or I2C

I have used with success MavLink on Arduino here:
https://discuss.ardupilot.org/t/avoidance-experiments-with-the-poc-and-benewake-tfmini/25277

You should read this good informative blog here as well:
https://discuss.ardupilot.org/t/mavlink-and-arduino-step-by-step/25566
and ask questions to the autor.