I suggest you first try to follow it yourself, maybe copy and play with A1 and see what happens, maybe copy aside parts of the formulas and see what they output

I suggest you first try to follow it yourself, maybe copy and play with A1 and see what happens, maybe copy aside parts of the formulas and see what they output. Then you can read the writeup I wrote in response to another commenter (the guy who wrote the original Apps Script VM).