Do public universities have different SAT/ACT minimums for students in more rural counties than for the large metro areas? I was told this is true because otherwise nearly all students would be from the large metro areas (areas with higher incomes and better schools), resulting in an incoming Freshman class that does not represent the state.
Most universities do not have hard minimums for standardized test scores. They will review your application in the context of your school–if in-state, the admissions officers will likely be familiar with the rigor and preparation of your school.
Have not heard of different SAT/ACT score minimums by county. However, there are related variations:
CSUs (not UCs) in California often have a local area preference, which is sometimes based on the applicant being from the same or nearby county as the campus. This may allow for local area applicants to be admitted with a lower eligibility index than non-local-area applicants. Currently, the eligibility index is calculated only based on high school record for frosh applicants (college GPA is used for transfer applicants); SAT/ACT scores are not used (although earlier versions of frosh eligibility index did include them).
Texas public universities automatically admit Texas resident frosh applicants with high enough class rank (top 10% for most, top 6% for UT Austin). This allows all Texas high schools a share of admission spots (within the portion that the automatic admit fill at the campus) proportional to their population.
US service academies require applicants to get appointments from representatives or senators, which ensures some level of geographic diversity based on congressional districts and states.
The College Board National Recognition Program includes a designation for PSAT takers from rural areas who score high enough. Some colleges attach scholarship money to CBNRP status.
Beyond that, state universities often have higher minimums for out-of-state applicants, though that is presumably not what you are asking about.