Ask HN: What makes for a top-tier web designer?

I am eager to know what is the process + knowledge + toolset employed by the web design teams at the Airbnb's, Google's, Twitch's, Invision's of the world...

I am a CSS/web design beginner, but want to become world-class over-time - the problem is: I don't know what I should study, so I don't really know where to start.

Any courses, talks, books, etc. that you all consider to be absolute musts or definitive guides?

Or recommendations on the best process for learning?