Can’t find any docs around the !! in:

const styles = require('!!file-loader!./styles.css');

>Solution :

In Webpack 4, the "!!" before a loader in the webpack configuration file is called the "bang-bang" loader syntax. It is used to override the configuration specified in the webpack configuration file for a specific loader.

Here’s the link to the official Webpack documentation on loaders:
https://webpack.js.org/concepts/loaders/#inline

you can find a section called "Inline" that explains how to use the "bang-bang" loader syntax to override loader configuration.