Is PostCSS a coomon package to use?

I’m taking a course on web dev workflow and I have a lot of questions about PostCSS. I know it is great as a autoprefixer tool for your CSS. Yhe course I am taking also installed style-loader, css-loader, postcss-simple-vars , postcss-nested, postcss-import, and postcss-mixins (I think that’s all of them).

I’m a total beginner uses SASS syntax, although that is at the top of my list for the next things I want to work on. Will it look bad if I have it in my package.json and webpack.config files, or is it commonly used.

Also, what is @mixin-content? Is it specific to PostCSS? Does SASS just use @content to grab whatever is in the mixin? Can you tell I’m a newbie at this stuff?

I’m just curious about the best practices when it comes to CSS/SASS.

Should this post go in the JavaScript section?