Why GPS makes outdoor flight almost easy
Outdoors, with a clear view of the sky, GPS gives a drone a remarkably simple way to know where it is: a receiver picks up signals from multiple satellites and triangulates a position accurate to within a few meters (and much tighter with corrections like RTK, real-time kinematic positioning). Combined with the IMU for orientation, this is enough for a drone to hold a fixed hover position, follow a pre-planned waypoint route, or return to its takeoff point automatically if it loses its radio link.
This is why most consumer and agricultural drones fly so confidently outdoors: GPS gives an absolute, drift-free position reference that doesn't depend on anything the drone can see. It's also why 'return to home' and geofencing features work as reliably as they do, since they're really just simple GPS coordinate math.