Recursion Order of Operations

Question

I am having some trouble understanding this recursion code:

class Main {
  public static void main(String[] args) {
    int[] intArray = { 20, 35, -15, 7, 55, 1, -22 };

        mergeSort(intArray, 0, intArray.length);

        for (int i = 0; i < intArray.length; i++) {
            System.out.println(intArray[i]);
        } 
    }

    // { 20, 35, -15, 7, 55, 1, -22 }
    public static void mergeSort(int[] input, int start, int end) {

        if (end - start < 2) {
            return;
        }

        int mid = (start + end) / 2;
        mergeSort(input, start, mid);
        mergeSort(input, mid, end);
        merge(input, start, mid, end);
    }

    // { 20, 35, -15, 7, 55, 1, -22 }
    public static void merge(int[] input, int start, int mid, int end) {
    if (input[mid - 1] <= input[mid]) {
            return;
        }

        int i = start;
        int j = mid;
        int tempIndex = 0;

        int[] temp = new int[end - start];
        while (i < mid && j < end) {
            temp[tempIndex++] = input[i] <= input[j] ? input[i++] : input[j++];
        }

        System.arraycopy(input, i, input, start + tempIndex, mid - i);
        System.arraycopy(temp, 0, input, start, tempIndex);
    }
}

Can someone help me identify the operation order in the code? When we call mergeSort, are we synchronously finishing the function or are we moving on to the next step and coming back to it later?