Some questions (possible spoilers)

[Captain Maxx Power]

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.

(SPOILER)It's made obvious throughout the game that Auron knows Sin is Jecht, and in some way Jecht can communicate with Auron See; "Are you sure?" in the opening cinematic. As for the latter part Sin is part of the construct that houses Yu Yevon, which in turn summons Dream Zanarkand, so Sin being able to reach it is no big deal.

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.

(SPOILER)I believe this was purely for aesthetic purposes. The idea was you knew something is attacking but you're not sure what. You don't really get a good look at Sin until the Mi'hen Operation. It all adds to the mystery that is Sin

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

(SPOILER)I'm not exactly sure what you mean. Sin is a construct of pyreflies much like the Aeons/unsent/fiends, so his appearance can be twisted and manipulated to the whims of it's creator; in this case Yu Yevon. If there is indeed a building on his back I for one haven't seen it.