The bodies of standard die cast vehicles are cast from Zamac
(an acronym for Zinc, Aluminum, Magnesium, and Copper alloy).
The vehicles are however generally composed of four distinct layers:
Body - Zamac
Windscreen - plastic
Interior - plastic
Chassis - Zamac
Although this is a standard configuration there are many variants and an increasing number of plastic bodies and chassis so be sure what materials you are dealing with before proceeding.
These layers are generally held together by one or two 'rivets' visible on the underside of the chassis. These 'rivets' are actually the flattened ends of posts on the inside of the Zamac body that extend through the layers and out of the chassis thus holding all the layers together in a fixed position.
Separating the layers allows:
Stripping paint with agents that would affect plastic layers such as the windscreen, interior and wheels.
Detailed painting of individual components, interiors etc
Swapping of components between different versions of the same vehicle, coloured windscreens, wheels etc