Big O notation O(NM) or (N^2)

I've been told the below code is = O(MN) however, I come up with O(N^2). Which is the correct answer and why?

My thought process: nested for loops plus if statements --> (O(N^2)+O(1)) + (O(N^2)+O(1)) = O(N^2)

Thank you

public static void zeroOut(int[][] matrix) { 
    int[] row = new int[matrix.length];
    int[] column = new int[matrix[0].length];
// Store the row and column index with value 0 
 for (int i = 0; i < matrix.length; i++)
   {
    for (int j = 0; j < matrix[0].length;j++) { 
       if (matrix[i][j] == 0) 
          {
            row[i] = 1;
            column[j] = 1; 
          }
   } 
  }
// Set arr[i][j] to 0 if either row i or column j has a 0 
for (int i = 0; i < matrix.length; i++)
 {
   for (int j = 0; j < matrix[0].length; j++)
      { 
        if ((row[i] == 1 || column[j] == 1)){
              matrix[i][j] = 0; 
           }
      } 
  }
}

Solution 1:

What does M and N refers to? My assumption is that it refers to "rows" and "columns" respectively. If it is so, then the equation is O(MN) because you loop through M number of N times.

O(N^2) will be correct IF rows and columns are equal.