Fatima tells Santiago that “The dunes are changed by the wind, but the desert never changes. That’s the way it will be with our love for each other.” Which literary device is this and what does Fatima mean?

I believe this is a metaphor, as it refers to one thing in order to show or suggest they are similar. The phrase means that, as how a desert can survive winds and remain steady, so is their love for each-other. No matter what happens, Fatima and Santiago’s love will forever be true-hearted and faithful. Hope this helps!