The sentences using the verb "ser" are highlighting characteristics or identity. Here’s why each sentence uses "ser":
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¿De dónde eres? - This sentence is asking about origin, which is a permanent characteristic.
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La chica es alta. - This sentence describes a physical characteristic of the girl, which is considered a defining trait.
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¿Qué día es? - This sentence asks about the day, which is a factual, permanent state.
In summary, "ser" is used for identity, inherent characteristics, and facts.