Game of Thrones S7E01 Explained

What is the meaning of Arya’s killing? Has Wun Wun joined the army of the dead? What’s going on at Winterfell? How does Cersei feel about Tommen? Why is Euron Greyjoy so fabulous? What does Sam discover at Hogwarts? How has the Hound changed? What does Dragonstone mean for Game of Thrones Season 7?