Material.io
From the https://material.io/ website (the official Google spec?), I end up here when I follow the path to access "Material Design UI" web components:
https://github.com/material-components/material-components-web/
Polymer Project
From the https://www.polymer-project.org/ website, I end up here when I follow the path to access "Material Web Components":
https://github.com/material-components/material-components-web-components
I don't understand the difference between these sets of components but would like to use the "official" Google web components for a web app I'm creating.