As a college and career counselor, I quickly learned that big schools like Illinois know the variations across states. For non-athletes, they look at the GPA and are looking for a student with consistent high effort and achievement, preferably who was also involved in the school community (clubs, sports, etc.) and/or a job or something similar. I went to a workshop at Illinois specifically where they took us through the application evaluation process so we could better help our students. The admissions counselors understand that a student coming from inner city Baltimore won't have the same opportunity to take AP classes, etc. as a student in Batavia. They take that into consideration. It isn't a perfect system, but they do what they can to account for inequalities and are constantly adjusting it. That's where regional reps come into play since they report back to admissions about what's available where; they're also the ones who call out the students in privileged districts who attempt to pad their GPA with BS classes.
As for athletes, we all know that comes down to who coach wants and if they pass the NCAA Clearinghouse more than anything else.
EDIT: ACT/SAT aren't seen as the equalizer they used to be. They're important and show potential (or lack of effort when a student has an outstanding SAT/ACT but a mediocre GPA), but given the inequalities in public education depending on the socioeconomic status of the community, schools are realizing that students in poorer districts are at a disadvantage there and are taking that into consideration as well. That's not just Illinois, that's across the board.