IntelliSense for JavaScript libraries and frameworks is powered by TypeScript type declaration (typings) files. Type declaration files are written in TypeScript so they can express the data types of parameters and functions, allowing VS Code to provide a rich IntelliSense experience in a performant manner.
Many popular libraries ship with typings files so you get IntelliSense for them automatically. For libraries that do not include typings, VS Code's Automatic Type Acquisition will automatically install community maintained typings file for you.
Automatic type acquisition requires npmjs, the Node.js package manager, which is included with the Node.js runtime.