Many sources answer this question by providing polish hackerrank (3rd place). But the answer to this question is more trivial. Polish developers love their job and treat it as a hobby. This results in a high motivation to constantly improve and become the best of the best. And we are not talking only about technical skills, but soft skills too. Curiosity to explore new technologies, to find new better solutions leads to mastery. When we add to this punctuality, openness for direct feedback and constructive criticism, proactiveness and good english level, you will receive an almost perfect engineer :)