those are two different questions, chris...
there's a cage, that's certified based on "standard" crash impacts (think of it like crumple zones based on speed)... i.e. a 8.99 or slower cage vs. a 7.50 cage, etc...
then the chassis that's designed to hold the body - think of it like how art morrison builds a chassis for a chevelle... there's enough of them going around that you can build a "basic" structure and you weld in or fabricate mounting points for whatever body you would like...
i'm dumbing it way down as there's YEARS of trial and error and geometry and CAD programming to make all this "look easy" to the average person, but that's how i understand it...
i'm sure others who know more can chime in depending on what your specific questions are... but you aren't going to learn chassis building overnight...