> Surely he didn't write this JS by hand

It seems parts of it are auto-generated like all those co-ordinates. But then some parts appear as if they are hand-coded.

I also noticed that other program texts are getting assigned to variables (example "line_vert_src"). Could someone please describe what's going on?

That explosion animation is absolutely mind-blowing. Goes to show what can be achieved if someone focuses their attention to a topic to understand it in depth and explain it at the same depth.