The desert shots were shot with a technique called translights.

You're right that they couldn't afford to shoot on-location for the desert scenes so instead they painted desert landscapes and scaled them up then printed them on 40-foot-high by 200-foot-wide transparencies and put bright lights behind them to illuminate the scenes.

They don't look anything like real life but they weren't really trying for realism as much as they were trying for artistry.

I think that when you view things through that lens, what they did craft was beautiful.