In order to install the server just run npm install in the root folder (where app.js) is located
Make sure you're in the server's root folder, and run npm start.
To download the files, connect to the host's IP address using a device connected to the host's hotspot and then navigate using the UI.
If you want to change the folder from which files can be downloaded, replace the value of the desktopPath variable in app.js with the full path of your target folder.
When you start the server, the host IP address is printed: if it says "not found", you forgot to turn on the computer's hotspot.