Best way to learn the built-in functions of a language?

I have jumped around to several different languages over the past few years, in and outside of school. I regret not focusing on one language the entire time and becoming an expert. Instead, I just know all the syntax and basics of each language. I seem to always follow the same pattern where whenever I try to learn a new language, whatever guide or book or video I read always talks about the programming elements themselves like if/else, loops, functions, classes, inheritance, abstraction, etc….
Best way to learn the built-in functions of a language?

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