They draw from who applies genius. Cathedral has an acceptance rate of >80% and with a typical freshman class being ~350 students that means they typically receive 450-500 applicants a year. A lot of the students who apply to Cathedral also apply to Chatard, Brebeuf, Guerin, etc and ultimately go there there paving the way for students who were initially waitlisted to gain acceptance. Simply put, if you apply to Cathedral, you’re likely getting in.
Any student in Central Indiana can theoretically choose to go wherever they want with open enrollment being instituted so save it.