You say it that way because in Spanish the adjectives and articles have to agree in gender and number with the noun(s) or pronoun(s). "Es la una" is a reference to a femininenoun or pronoun. That being the case, uno, being a masculine singular adjective, is incorrect, because it fails to agree in gender, even though it agrees in number (singular). Another reason is that, when telling time, they are las horas, the hours, and they are all feminine: la una, las dos , las tres, las cuatro, etc.
Assuming Lexy is a person, it depends on whether Lexy is male or femal. "Lexy es la una" if it is a female, "Lexy es el uno" is he is a male.
Ella es una maestra
You could say "es la una y treinta" or "es la una y media".
thirty one in spanish is said as treinta y uno (the y in the middle makes the eee sound)
Es la Una, that is not the way to say one o'clock in Spanish it is la Una en punto
Ella es una chica mala
you are a mean professor... but "es" would be "eres" instead.
es una galleta
Es la una
"es uno" but it depends on the situation
Una fuente primaria es un documento original que contiene información de primera mano, como cartas, diarios o informes de investigación. Una fuente secundaria es un documento que interpreta, analiza o resume información de fuentes primarias, como libros de texto, reseñas o artículos de revisión.
You would say "esta es una nariz."