Too Long; Didn't Read
You’ve gotten a little comfortable at work. Code reviews are shorter than your code. And you've shipped some stuff. So you’re motivated to branch out and learn more. But the sheer volume of resources is mind-boggling. Focus on these 5 areas: Low-level/Library Design, System Design, Concurrency, Design Patterns and General Coding Practices. You should be able to find equivalents for other languages. Most design concepts are language-agnostic anyway. You can expect a range of answers based on how skilled the interviewee is.