Derivative works, evergreening and copyright freedom to operate
In William’s post, he explained how by copying a public domain work which is a derivative work, you may still infringe copyright in the original work (if copyright still exists). (William also explains some US-specific issues relating to registration of copyright , which are worth a read.)
The point is that each work has its own copyright protection, regardless of who made it, where or when, etc, So, how to avoid copyright infringement?
Don’t copy, get a licence, or work your way through the quagmire to sort out what copyright, if any still exists, and what it extends to.