This is what you say when you are agreeing with, or confirming, something.
So is it, So does it.
This means 'It is too (also), It does too (also). You're adding something to what's been said, rather than confirming something that's already been said.
A: What have you done to the chair?
B: I've painted it. Now I'm all blue, and so is it. (Adding)
A: Yes, so it is. (Confirming).
A: Why are you and your cat drinking milk?
B: I like milk and so does it. (Adding)
A: (Watching the cat drinking) So it does. (confirming)
(We'd normally talk about our pet cat as 'he' or 'she', but it's just for example).
Student or Learner