Hypothetically in the context of "What's your favorite restuarant to eat food? If they have food, I'll eat it.". Does the second sentence mean they would eat food at a single resturant or multiple restaurants?
When the person responding says, 'If they have food, I'll eat it', they are saying that it doesn't matter what restaurant they're visiting. If they're in a restaurant, then they will eat the food there. They're ignoring the question about which restaurant is their favorite and implying that they don't have a favorite. They're saying that they like any restaurant because what's important is not that it's a good restaurant, but that there is food there they can eat.
You might be confused by the pronoun, 'they', which is normally a plural pronoun. But in this case it is not referring to multiple restaurants. 'They' means the people running the restaurant where the person is.