RTS game?! made In a week? No way! Just played and rated yours, keep up the amazing work. If you are down to detail how you implemented all the mechanics I'd be down to listen XD
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.