The sentences are using the verb "ser" for the following reasons:
-
¿De dónde eres? - Origin: This question is asking about someone's origin or where they are from.
-
La chica es alta. - Characteristic: This sentence describes a characteristic of the girl, specifically her height.
-
¿Qué día es? - Date: This question is asking for the current day, which is a form of identifying a specific date.