most common drone autopilot software stack for ARM

Yes there are other stacks (eg, librepilot) but ardupilot is the “full-featured” autopilot code, with a host-based stack such as mavconn or ROS for higher level tasks. If you’re looking at drone projects, there are a few publications/posters* that give an architecture/context overview for a proper mental framework; the specifics are left as a student exercise, including finding the report from last year’s beaglepilot project.