Ask HN: Efficient Usage of an API, Framework or Language

All of us use some kind of framework, API, language built-in libraries or 3rd party libraries while developing software. These software bundles provide us tools for reducing complexity, write more efficient code and apply DRY. But, what is your method to traverse a library and find out suitable methods for your need or how do you know the task in your hand is already done by the library you use?

To give an example, I wasn't aware of the StringJoiner class in Java and I've done all the concatenation with delimiters using StringBuilder all this time until I saw it while studying a book. It creates more readable code and feels more smooth, I wish I've used all this time.

PS: I am sorry if this is a silly question.