⚠️ Like for importing JSON, importing a CSS file is made possible because you are using create-react-app. Under the hood, create-react-app has a webpack configuration that allows you to import CSS files.
If you inspect the source of your app, you will see that the CSS is inlined in the page. ⚠️ You are using plain CSS here, it is not CSS modules, so if you use same class names, you will have collision.