This section expands upon the general environment info
BetterDiscord gives plugins access to the Node.js bundled into the Discord desktop application. This means anything you can do with Node.js can be done inside of a plugin. There are some caveats, especially when it comes to using modules from the npm ecosystem because Electron applications require many packages to be compiled specifically for use with Electron and for that specific platform/OS.
The standard library of Node.js is fully available inside of BetterDiscord. For instance, if you want to load a file in the current directory you can just use the
const fs = require("fs");
const myData = fs.readFileSync("myfile.txt", "utf8");
Though this guide won't be giving a tutorial on the Node.js standard library--their official docs do that--you will see example usages throughout.
BetterDiscord provides an API for plugins. The guides here show how it's used and the API reference section has an exhaustive list of what's available. The API exists as a global and provides several utility functions relevant to plugins. This includes data storage, UI rendering, notifications, and utilities to explore Discord's internals.
Inside of this environment, BetterDiscord provides access to Discord's internals via searching their modules. Understanding and using these modules is a task left to the developer. But the advanced guide provides some insight on how to get started. Searching through and using Discord's own modules are some of the most important skills for building complex plugins.