engineering of car-bodies

As most of us here are aware that passenger cars do not have chassis frames, unlike the heavier vehicles like trucks and lorries.

but the chassis and the body are both integrated as one single unit, and it resembles something like a shell (without the rest of the components).

now then how are these units fabricated ? what kind of process is used ?

Car bodies have two distinct portions, one is the crumple-zone, which is the frontal portion of it and the one which houses the passengers is called the passenger-zone.

these two are designed in such a way, that the passenger zone has the maximum stiffness and the crumple-zone the lowest. this is done to ensure that during a crash, the crumple zone takes up all the impact , and self-destructs , while the passengers remain protected .

also how can this be achieved ?