When the state of Amazonia was created by Hualpa's coup d'état in 2034, all the non-awakened population living in the interior was forced out towards the coast. Instead of occupying other major cities like Fortaleza, Salvador, Recife or Porto Alegre, most of that population moved over to the Rio-São Paulo axis.
So much people in a space (relatively) made both cities sprawl towards each other, making the Metrópole as we know.
Its a known fact that, even if forced together, the people of the sprawl tend to identify themselves either as cariocas (hailing from the old Rio de Janeiro) or paulistas (hailing from the São Paulo side). That's mostly an expression of the old rivalry the cities always had, and which is taken by different people with different degrees of seriousness.
Metrópole is a 'free enclave'; basically, Amazonia lets anything go in there business-wise, as long as it stays within city limits. Little of its internal politics have been explored, but it can be assumed it's not that different from other city-states like Seattle or Berlin. It IS of note said city limits doesn' grows to the sides.
You can only get to the Metrópole by plane or boat. The highway network that criss-crossed the old Brazil is now useless, thanks to the forests' regrowth.
The only listed population quote for this, the 'largest city in the world' is from the Sprawl Survival Guide - "20,000,000". Given the 'largest city' claim - (compare New York City at 44,024,620) and the fact that Rio and São Paulo (circa 2012) have 25,000,000+ inhabitants in their combined metropolitan areas - this is clearly wonky. Further, Hualpa and company drove inhabitants from throughout former Brazil here, so even 200 million may be conservative (it is an estimate). Peter Taylor, line developper at the time, attributes this discrepancy to a typo. The figure of 200,000,000 reprised in later books is the one he intended to publish in Sprawl Survival Guide.