How case law can be used in US when every state has a different law?
Recently I read about common law and how it works. Two questions came into my mind:
1- How case law can be used in US when every state has a different law? I mean some states have their own rules, then how can a case law of a different state be used in the other one?
2- After the passing of new laws, how the new cases can be judged? based on previous case laws, or the newly passed law?