Hi Olga "It" is a personal pronoun, a word like he or she, but used for objects and animals. The object could be anywhere. "This" is more specific and applies to one specific and nearby object or animal. The plural form of it is they. The plural form of this is these. I hope this helped.

Basically it means the same thing in certain situation, like your example, but it can change when you say "it's me". It's not the same that "This is me". And about the plural thing it would be: It is = They are It is a mouse = They are mice