[SPOILERS] Fate/Zero ending...?

Ok.. a lot of questions, but first I want to know is my confusion coming from me watching the prequel first? Or is it actually confusing to understand?

1.) Why are Kirei and Gilgamesh alive? Did Kirei wish for life? Or did Kiritsugu wish for something?

2.) Why does the grail destroy the city?

3.) What happened to Saber?

Note: Zero is the only Fate anime I've watched yet, so I know some of these questions spawn from that being the case. I was told there would be a scene that you can't understand unless you see /stay night. I'm assuming the end is that scene.

Sorry for not understanding this and immediately asking this sub.