Related: Dig into your language's source code and code of major libraries! Your standard lib is probably the best examples of idiomatic code and efficient code that you're going to have access to, written by the authors of the language itself. You'll notice bugs, inconsistencies and poor docs which gives you opportunities to contribute upstream, at the same time making you read more and learn how particular features work under the hood, which might give you new perspectives in your own code.
In Java, this might be java.util.Collections package, or java.util.Concurrent. or in C++ that might be Boost. In Elixir, I start with Kernel.ex[0]
https://github.com/elixir-lang/elixir/blob/master/lib/elixir...