I think there’s a correlation between selective colleges and high average GPAs, but it’s not causally related. The types of people selective colleges choose just so happen to have high GPAs, but I don’t think they choose people simply because they have high GPAs. GPA is one of many things that makes sure that you’re qualified enough academically; of course, class schedules, test scores, ECs, essays, etc. also matter.
Depends on the high school. I could have a 4.0 at a bad high school but be worse off in terms of college acceptances than a person who had a 3.8 at a very rigorous school. Having a high GPA/Class Rank/Course Rigor is one of the many hurdles you have to get past to be admitted. You also need compelling activities, glowing teacher recs, awesome essays...
Yes
Well yes, but high GPA in challenging courses is best.