One of the common differentiators I see between a senior engineer and a junior engineer is the depth and bredth of their developer toolbox. Generally when I think of a developer toolbox, I think of a collection of patterns that a developer may apply to solve a problem.
Frequently I find that the tools in my own developer toolbox tend to be honed and refined by debugging, but often I can encounter new patterns while exploring a new topic or a new problem space, additionally reviewing my teammates pull requests has been an incredible opportunity to add things to my own developer toolbox.
The actual tools within a developer toolbox usually vary from person to person and could be entirely different from the rest of the tools as well, and while you may never work on the same exact thing a second time, these patterns can still be applied to new situations.
What things are you doing to build your developer toolbox?