What are the literary devices used in Warcross by Marie Lu?

Marie Lu uses various literary devices throughout Warcross.

I look for the break in the pattern, the nail that protrudes.

Lu offers a nail sticking out as a metaphor for a break in the code. This comparison helps us visualize what Emika looks for when hacking.

Finally, they end with a slender line of words that run along my left collarbone, a mantra Dad used to repeat to me, a mantra I recite to myself whenever things get too grim.
Every locked door has a key.
Every problem has a solution.

A mantra is a commonly repeated phrase, which Lu demonstrates through repetition —she repeats "a mantra" so we understand how it has transferred from her father to her, and "every" within the mantra, showing us the idea of being able to always find a solution. The locked door and key also serves as a metaphor for problems in life.

Sure enough, one of the magazines features him prominently: a tall young man lounging in an office, dressed in dark trousers and a crisp collar shirt, sleeves casually rolled up to his elbows, his face obscured by shadows.

This description features alliteration. The letter d is repeated in "dressed" and "dark" and the fricative sound of a c is repeated in "crisp," "collar," and "casually."

But sometimes, people kick you to the ground at recess because they think the shape of your eyes is funny. They lunge at you because they see a vulnerable body. Or a different skin color. Or a different name. Or a girl. They think that you won't hit back—that you'll just lower your eyes and hide. And sometimes, to protect yourself, to make it go away, you do.
But sometimes, you find yourself standing in exactly the right position, wielding exactly the right weapon to hit back.

This significant quote has multiple uses of repetition ("sometimes," "or," and "exactly"). It's also an extended metaphor.