The "IT girl" thing literally arose at the peak of OncexBlink fanwar around 2018 when Twice was clearly dominating Blackpink in Korea in all areas and blinks needed something to latch onto like global popularity and individual popularity. So it started the trend of "global group", "local group" and the famous "IT girl".
What is an it girl? Literally everything Jennie was, the term was coined so that it fits Jennie but not any Twice member. At first it was just "popularity", "going viral" but since Nayeon, Sana and Momo would frequently go viral a gatekeeping mechanism had to be invented. So the last element was added into the mix "having brand deals". Why? Because everyone was aware of the way JYP handles Twice and everyone knew Twice would never get solo deals.
Then people started giving the term to other non Twice idols that fit the description like Irene or Hwasa.
I was there the whole time watching this thing evolve. It was nothing but a term invented purely to elevate other groups over Twice in a way and now seeing 4th gen stans embrace it and apply to their faves gives me a weird feeling.
Like not all groups are operated in a same way, stop latching onto dumb terms