How CSS Files are bundled in GatsbyJS

Question

I wanted to play with all different types of styling methods in GatsbyJS. In my project, I have three types of implementing stylings, global CSS, CSS Modules and CSS-in-JS(styled-component). Everything works as expected. But, when I build the project with gatsby build and open one of the index.html where I did CSS-in-js I see that there is a CSS file in data-href tag inside. When I open it, It contains all the CSS stylings of my project. But I didn't even import it to the component where I did CSS-in-js. Why would something like this happen? Why stylings from module.css are being referred inside this file.