It's a bitsa (bitsa this, bitsa that).
The main chassis is unique to the Multi and it's a spaceframe. Looking at all the structure underneath, a pretty hefty one too. The Fiat-Alfa specialist I sometimes go to (ex-Fiat guys) say that it's built like a tank and the basic structure is very tough.
The front and rear subframes are also unique, but have a lot in common with the Tipo and Bravo/Brava, though a bit wider. In parts, it's also similar to the Marea.