I have two questions related to your answer.

How did you find the logic to calcuate the value of count? Is there someplace I can read about it more?

count += mid + 1 - i;

Second. I made a mistake of swapping aux and arr in the following code based on the function definition and I was getting my answer wrong. Why are you switching? What is the reasoning behind it?

count += countInversions(aux, lo, mid, arr);
count += countInversions(aux, mid + 1, hi, arr);