Yes, it’s college specific, and not all schools are transparent about what they use.
One of the “tells” though if a school is using weighted is if the reported GPA is above a 4.0.
Keep in mind that rigor will be assessed no matter how a school is reporting their GPA range. And that GPA averages could differ even within a university if students apply by major/college.
Purdue specifically uses UW: “For reporting purposes, we get our middle 50% GPA range by converting grades to an unweighted, 4.0 GPA”. Source: Freshman Class Profile - Undergraduate Admissions - Purdue University.