سماء and سماوات ,السماء الدنيا

I actually have 3 questions. They are:- - First question: What is the meaning of the word سماء in the context of the Qur'an? - Second question: Can the word سماوات be used as a replacement for the word سماء ? - What are the difference between the words سماء and السماء الدنيا? If there's any problem in my question please inform me. Thanks!