In this car the Greek is equivalent to the Spanish "qué tienes debajo de la camisa" nearly word by word. And yes, it's slightly ambiguous, but we'd never say "debajo de TU camisa" in normal circumstances. However we would translate it "under your shirt" as we'd assume it's the second person's shirt. I've got the Greek right because I know how literal Duo is, but in normal conversation that's how I'd interpret it
As with most of our words "yes, they are for practice". But this one could be used for example: "I see something new in your drawer, what do you have under the shirt." Silly, maybe but with a need to create thousands of sentences under strict vocabulary and syntax restrictions ...that happens.