I was wondering how universities calculate GPA / evaluates the high school GPA? Do they look at the GPA from each year individually or do they calculate the average GPA from all four years? Also I was wondering if there is a section on college admission applications where I can mention some errors in my grades(like an explanation why the grades are what are shown) because I caught the errors after they were submitted?
It depends on the college.
Also, colleges with holistic admissions reading will have admission readers look at your record holistically to note things like upward or downward trends and the like in addition to any (re)calculated GPA that the college shows them when making their initial admissions evaluations or scores.
Every college calculates GPA differently. Some schools weight, others don’t. Some schools use acadcmic subjects only, others can include some non-academic courses. Worrying about how a college calculates GPA is not a good use of your time and energy because it is out of your control. Just do your best.
And any explanation of material uncorrected errors in grades (hard to believe a flat out error can’t be corrected retroactively) should come directly from your guidance counselor as part of his/her recommendation. It would not be believable coming from a student or could come off as whining.