I recently started a new FFX game and some things I still don't get..

1.) What was Auron doing on the bridge in Zanarkand, reaching out to Sin? It seems like he was almost calling him. I always wondered how Sin got into Dream Zanarkand.
2.) Why is Sin a sphere of water when he is attacking Zanarkand? And then he becomes like a huge sucking thing, way bigger than normal. You don't see this when he is attacking areas in Spira.
and
3.) This is near the end of the game. When fighting Sin you see what looks like ruins of a city on its body...what is that?
The plot of FFX has always confused me