TL;DR: Mostly walk off, sometimes downclimb. In his interview with Joe Rogan, Alex mentioned that he is generally able to down climb anything he solos. Wow, that makes him more of a bad-ass than I had ever realized.

How did Honnold get down?

He rappeled down with an ATC which he borrowed from the camera team – so he did not actually climb back down where he came from. They didn’t show these scenes in the movie, but he talked about it in a Q&A.. here he also states that they had a 1000 foot fixed rope and that’s what he used.

How does free solo get down?

Climbers free soloing can get down the same way any other climber gets down. Walk off – at the top of the climber, the climbers can hike back down (Class 4 and lower) Rappel – Bring a rope up with just for the purpose of descending with it. Downclimbing – Climb down the cliff.