What is the city on top of Sin?

Sin has a city on its head, what is this city and where did it come from? We know it's where the party ends up after Lake Macalania, but where did this city originate from and what is it?

Pictures: 1, 2

Solution 1:

It's never brought up in the game, but it's assumed that the buildings are from Zanarkand, given Sin's relationship with that city.

Thematically, Sin is designed to represent natural disasters:

Yoshinori Kitase created [Sin] to represent the kinds of calamitous disasters that exist in the real world, such as earthquakes and typhoons. [...] Having ruins become a part of Sin's physical form helped to visually cement the idea it is an "unimaginably ancient monster on an unprecedented scale".

Thus the buildings are also symbolic, a representation of the way natural disasters cause ruin for the things humanity builds.