Their singular form is it/he/she, and they are the plural form of the third person. There is no gender difference, we can translate it into them, them or them. They are usually used as subjects in sentences, usually after the noun it refers to or before the word it refers to in special cases.
Personal pronouns have nominative and accusative forms, and personal pronouns of the third person also have gender changes. The nominative case is mainly used as the subject of a sentence and can also be used as a predicative in formal style; The objective case is mainly used as an object, and it is often used as the predicate of a sentence in spoken language. In spoken English, if the predicate and other components are omitted, the personal pronoun accusative can be used as the subject. For example:
He has read this novel. Me too. He has read this novel. I have seen it, too.