> Read through language references like the Java API specification, Python
  > Language Reference, or Ruby Documentation. Know the data structures your
  > language gives you: what methods are available for an array, a dictionary,
  > a stack, a queue. Read through the calls you can make for an API and the
  > parameters you can pass to a function.    
  > 
  > Try to commit these things to memory if you can.

Please do not just read through the Java API specification. It is enormous. A lot of it is irrelevant to almost everyone (the first entries are java.applet.* and java.awt.*). A lot of what sounds relevant is often implemented better and more usefully in third-party packages. This is BS advice.

Read the docs for types and modules you see in use, in your codebase. Do an occasional code dive into the standard library and poke around in it. Do not just read through the Java standard library.

After 15 years of engineering... I'm still insecure. So are most engineers. I wouldn't tell someone to stop being insecure. I'd let them know it's totally normal to feel insecure.

My tip is to ask chatGPT about things you might ask a senior dev or mentor and see what comes out. Questions about your stack, about specific problems, libraries, npm packages, algorithms, etc. Don't just assume everything it says is correct or the best option, but it can't hurt to ask.