We usually say "that's it" in English. I was curious to know can we also say "it's it"? Is it used in English?
No, "It's it" isn't a set phrase like "that's it"" and, actually, isn't possible under any circumstance to mean anything like "that's it". "That's it" is a set phrase that is "used to say that something has ended" or is "used to say that something is correct"
However, it is a legitimate phrase and may mean "It is what it is".