[swatcoder]

The absolutely most fundamental thing to master is to read books about the concepts, practices, histories and philosophies of the craft.

It's become very popular to learn how to assemble code by following step-by-step tutorials and watching YouTube demonstrations. It does get the job done for a lot of people, but it usually doesn't develop very strong or deep foundational knowledge. As a result, people who don't make a point to leave that world generally end up stuck under a low skill ceiling.

So whatever suggestions you find in other replies here, favor books (and perhaps deep courses) over anything more shallow. And among those books, favor those that are heavy on prose and explain things over those that just guide you through doing things. And then when you get the books, sit down with them and really work through them.