No, not all cameras use the same chassis; they use a lot of interchangeable components, though.
I think the ZI is more different in terms of chassis. You can see that from the corners, they are considerable more rounded on the Bessa. They use interchangeable components, hence the similar layout of controls and more or less the same shutter. However, the controls are different, too; a placement of the rangefinder window as on the ZI would be difficult to do on a Bessa.
Basically what Cosina has is a toolbox of components. Then a customer comes and says "build me a camera to these specs", and they start picking components and building the camera. The ZI is actually a pretty extreme case here; from what I understand the entire body shell as well as the rangefinder are new developments over the older Cosina cameras. Other components are the same, though.
Philipp