Node.js comes with modules that are built-in (that we didn't need to install from npm). Some of those we used already, related to writing a server (http) or working with file paths (path) and even to print in the terminal using console.
Accept input from the command line in Node.js
Reading a file line by line in Node.js
Introduction to the fs Module in Node.js
There are a lot more modules, for all sorts of different use cases. I suggest you look into this series of exercises to get some practice.
PM2 is a process manager for node.js applications. It allows to monitor the memory usage of a node.js app, to run multiple instances of an application on the same server, and also to handle auto-restarting if the app was to crash.
In order to run a server, we have used express which is by far the most popular framework, but you might also encounter other frameworks that aim to help node.js developers to write server code, with different approaches.
Whether it is to solve a problem that other developers face (and give back to the community), or to share code (building a library of functions) with other team members, you can also create your own package and publish it on the npm registry.
Creating (and publishing) your first npm package
Contributing packages to the registry on docs.npmjs.com