There were no drones or CG back then. How was the "flying in" effect filmed?
According to the website Sound-of-Music.com it was shot with a helicopter mounted camera.
The opening scene that became one of the most famous in film history was filmed on a mountain about 10 kilometers into Bavaria. The scene was shot from a helicopter and had to be perfectly timed, so one of the crew members hid in the bushes with a megaphone and yelled "Go, Julie" when the helicopter was in the right position. Julie Andrews had great difficulty standing upright because of the jet helicopter´s strong downward drafts.