Create a Python function that takes in a 2D array as input and outputs the sum of all its elements. The function should not use any built-in Python functions or libraries for calculating the sum. It should also handle arrays of any size (not just square arrays). The function should be named "sum_2d_array" and take one argument, the input 2D array. Your code should include comments to explain your thought process and each step of the solution.
This prompt doesn't seem malicious.