There's no simple answer because it actually depends how the developer has arranged their repository (which is just a folder full of files).
In many cases, developers will publish the Node.js "exploded" plugin folder, where the tiddlers making up the plugin are separate files. That form is easiest for other Node.js developers to consume.
In Danielo's case, he's included an HTML file that includes the built plugin, so you can download the ZIP of the repo, open that file and then install the plugin with drag and drop.
Still other developers might use the GitHub "Releases" feature which allows you to upload arbitrary ZIPs of stuff.
All of which complexity is why we now have a plugin library :)
Best wishes
Jeremy.