Time complexity single loop with two variables

Question

What will be the time complexity of below code and why?

public static int[] Shuffle(int[] nums, int n)
{
    int len = nums.Length;
    int[] final = new int[2 * n];
    int counter = 0;
    for (int i = 0, j = n; i < n; i++, j++)
    {
        final[counter++] = nums[i];
        final[counter++] = nums[j];
    }

    return final;
 
}

If we will have two loops as below then it will be considered as time complexity of O(n^2)

for (int i = 0; i < n; i++)
{
   for (int j = 0; j < n; j++)
   {
   }
}

Answer 1

Complexity is O(n) because the cursor is looping from i = 0 until i = n-1. Number of variables doesn't matter when it comes to time complexity. (there is space complexity as well) However care,

for (int i = 0, j = n; i < n; i++, j++)

is completely different from

for (int i = 0; i < n; i++)
{
   for (int j = 0; j < n; j++)
   {