Commands that transmit to autopilot

I am a student and I am trying to understand how QGroundControl communicates data to the autopilot. I understood the structure of the mavlink package (, but I don’t understand how the data is transferred in payload. I am interested in creating a mission, launching a mission, arm/disarm, takeoff/land, and similar.
What is this data? Command codes or their names or something else? Where can you read about it?
Sorry if this question is stupid, I’m just getting started.