“It” is used to describe an object that both parties already know what you are talking about.
“Them” is the object pronoun for the subject “they”. The object pronoun is the object of a verb and usually receives an action.
The main difference between “it” and “them” is that “it” is for singular nouns and “them” is for plural nouns.
I lost my key. I can’t find it.
I lost my keys. I can’t find them.
We use “It” to refer to objects that both people understand through context of a previous conversation.
We do not use “it” for people.
Where is my guitar? I can’t find it.
What can I do with this T-shirt? Put it in the bin.
In the first sentence, “It” is the guitar and in the second sentence, “it” is the T-shirt. We use the pronoun “it” to replace the noun(guitar) so we don’t have to keep repeating “guitar”.
It can also be the subject of the sentence.
It is a beautiful day.
It is correct.
The noun is often implied and never explained because it is obvious from the context. In the first example, it replaces “the weather” and in the second example it could replace the exam question but we need more context to understand. In this context is necessary to use “it”.
“Them” is an object pronoun. We use object pronouns for people or things that receive an action. “Them” is the object pronoun for the subject pronoun “they”(the person or thing that performs the action).
We can use them with people or plural things:
They are so bold! Can’t you suspend them?
Where are my keys? I can’t find them.