Those wide shots sure seems to be real outside shots of a plane ( or a model plane?) but the reaction shots of Cary Grant noticing the crop duster plane seem to be either Cary Grant in front of a studio backdrop or him in front of a matte screen.

The reverse angle of Cary Grant ducking from the plane are almost definitely studio shots… then the wide shot of Cary Grant getting up off the ground looks quite real.  Why not shoot the close up on the same day as when they shot that wide shot?

Then Cary Grant runs to the road to wave down a car. Im sure thats really Cary Grant really outside on a real road, right?

Cary runs from the plane and camera is dollying with him, thats real, right? Then the close up of him hitting the ground is a studio shot?

Grant runs across the ground and into rows of corn stalks, thats real right?

Somebody, maybe not Cary Grant but a stunt double, runs through the corn but we don’t see them.  Cary Grant is next seen in close up… studio mock up of a corn field?

The plane buzzes the corn field, close up?

Cary Grant ( or a  double?) flees the corn field and runs towards the truck.

Cary in medium shot waves at the truck and looks really artificial.

A real truck and surely a stuntman falls under the truck.

The real Cary Grant is under a real truck.. but it looks to be in a studio.

The plane hits the truck… once you look it is surely a well done set of models and model environment.

Then I think its a real truck and real plane ( really on fire) and….. Cary Grants character with his back to the camera so it might be a stunt man flee the scene.

Then the real cary grant flees the burning scene which seems to be really outside, but was Cary Grant added in on top of the burning truck and plane footage?  If its fake it works quite well.