About "public domain":
Public domain is the default. In other words, anything that isn't explicitly protected by copyright is in the public domain.
For a real-life example of how this works, I'll use my favorite recipe for Reine de Saba or "Queen of Sheba" cake. A list of ingredients and a procedure for assembling them isn't protected by copyright, but Julia Child's written description of how to make the cake in Mastering the Art of French Cooking is protected by copyright. In other words, it isn't the recipe that's protected, it's the creative work that includes that recipe.