So if God is eternal and all things are NOW for him, then his act of creation is eternal and always now. Since God is pure act then he couldn’t have decided at one point in eternity to create the universe. Eternity is timeless (hence God being outside of time) therefore there is no potency to decide to do something (God is pure act). So God must have eternally chose to create. Therefore the universe eternal for him.
I mean for us, of course, the universe isn’t eternal. I imagine it like this: an author has just finished his book. For the author, in a sense, all things in that story are present for him. He knows the beginning and end, the decisions, the causes, the characters, etc. For the characters, however, their lives are unfolding from moment to moment. Whereas for the author all moments are present to him.