I think this is done by CSS Post-processors like PostCSS. Just as post-processors removed un-used classes; it would be to get a log or manually run it to get how often a class is used to tell you which components powered by that class is used in product/dev or wherever.

I’ve been away from development for quite a while but I remember using simpler tools before the days of easy-to-use post processors.

Here is how to use Grep to look for occurrences of specific classes, elements, etc to tweak/track or do something with it. https://brajeshwar.com/2013/hijacking-developer-tools-optimi...