This! Could you please share your insights as to how to improve your code-reading skill? Besides the obvious, "read more code". For example, how to select which sources of code to read from, any tips/tricks you've learned in your process, etc?
This not only lets you read more code, but also lets you pick up knowledge in areas that are often taken for granted (no need to think about X, there's a library / plugin /framework for that).
Every piece of software has bugs and limitations. When you encounter one, go figure out why it is the way it is, and maybe even fix it.
If all your dependencies are so perfect that this never comes up (hah!), you can always pick a project you use and like and look at their open issues. Begin trying to solve one of them.
The biggest bottleneck I see that prevents people from being able to do this is their code-reading skill.
With practice you can read and understand codebases much, much more quickly than most people (who are so intimidated they never even try). Once you can do this, you can very quickly pick up (1) the rich details about how the system works, (2) the ability to extend the system and fix bugs in it, (3) understanding of what is idiomatic and what is not.