There are a lot of ways to return a collection of things. List, IList, IEnumerable, ICollection. Which collection type gives the best performance? Which one should you be using in your code? We benchmark them all to figure out the tidbits of what the costs of each are.
Pure functions make your life and code better. Every function you write is either Pure or Impure. It's an important distinction that is never really taught in school or at most jobs. Learn about how writing pure functions can make your code easier to test and free from side effects.