Awesome, thank you! I did it in 5 days... It hasn't been an entirely sane or healthy 5 days. I have been eating, sleeping and drinking game development for those 5 days :D
Nope! It is a flat array with a simple formula for the index:
index = row * number_of_columns + col
And the array stores references to nodes of a custom class called "BuildObject" which inherits from Node2D. It's a neat way to store all of that information.
I will probably cover it on my YouTube channel eventually, I think it's a really interesting topic.