How did they do the shots w/Redford + Sarandon etc. ?

They DID it, that's how. Redford was doubled by an amazing man, Johnny Kazian, a former trapeze performer who later flew in the Korean War and was shot down in combat. His arm was shattered in the crash, and he received no medical attention from the North Koreans. As a result, he was partially crippled in his hand and arm and couldn't return to the trapeze when the war ended and he was freed as a POW. Not to be deterred, he started wing walking since, "I could do that with only one good arm."

For the wing walking scenes, Redford himself wore a safety cable and that's him you see exiting the cockpit in at least one scene in the sky. Other times Kazian wore a mask that was about a quarter inch think but that moved when he smiled or spoke to make him look like the film's lead actor. Kazian's career spanned four decades and he's only relatively recently retired. That's him making the plane to plane transfer without a parachute.

Sarandan's stunts are mostly done on the ground, shooting at up angles. IIRC at one point the plane is mounted on a moving truck to get angles of the stores flying by. The wide shot of the street scene uses a dummy. Other scenes seem to have employed a female stuntman.

But the striking thing about the entire film is that so much of the flying sequences and wing walking is 100% real.

As a fan of aviation and the son of a barnstormer, I love this film and lament that todays films are so clearly fake. I almost cried when the closing credits to THE AVIATOR came up, and it was clear that exactly zero pilots were employed on the project.