They and them are always used in place of plural nouns or noun groups in the third person. However the fundamental difference between the two in grammatical terms, is that they is a
Is it correct to say it is they?
When referring to a generic person whose gender is unknown or irrelevant to the context, use the singular “they” as the pronoun. ... If a person uses “she” or “he,” do not use “they” instead. Likewise, if a person uses “they,” do not switch to “he” or “she.” Use the pronouns the person uses. Kai is a nonbinary person.
Is it they or it?
Unlike the singular "it" which always implies a non-person, the plural pronouns "they", "them", and "those" do not. The plural forms should be used for any plural noun, whether or not it is a person. There aren't actually plural forms of people-specific pronouns.