With [email protected] and higher, adding scss files are easier with just npm install sass and rename the files from .css extension to .scss extension.
Find the official page here for more details: Adding a Sass Stylesheet | Create React App
It’s also worth mentioning here that Create React App is no more recommended by library authors so better to consider other options provided in React site.
For earlier create-react-app versions, we need to `npm run eject` and do modification with exclude and loaders. However, ejecting the app is not recommended.