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?
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.
From my understanding it was a shadow of the inter dimensional space Sin creates along with the Faiths since look how far you half to walk from where the Fahrenheit lands in the Sea of Sorrow all the way to Dream's End.
We see The Nucleus in the aftermath of Operation Mi'ihen and in it we see Luzzu along with a bunch of Crusaders and Al Bhed running around, after Luzzu walks away we see Bahamut Faith appear (that child who became the Faith).
the city on Sin kinda looks like City of Dying Dreams and i think also beneath Macalania the party is sitting on that area of Sin which explains the ruins, the ruins we see in the images you provided being a shadow of inter dimensional space would explain why the ruins the party was in Macalania wasn't as big, that's just my understanding however.