I also didn't understand this, if they were using NextJS, why do they have a custom webpack config? My assumption is that there is a separate web app that does the drawing and coloring, they bundle it into a script.js and include it in their nextJS page.

Yes, this is correct. All the canvas stuff has nothing to do with React, so I bundle it all separately. In fact, I don't even load React on the browser, I just render the initial page with it on the server then just work with the DOM from there on out. It greatly reduces the amount of JS shipped to the client, and since 90% of my code was going to be Canvas related, it's not much overhead to add a few click listeners for various DOM nodes

Ah true, my bad. Though it's still relevant to mention, because usually when people hear "using Webpack" they also assume "using Babel". Bun is doing both the bundling and the transpiling, so it should be compared to the bundler + transpiler combination