How do they bend metal plates and how thick are the plates?

[Squidly-Diddly]

Do they use a giant set of rollers or 'pounders' as I've seen used to make custom curved auto fenders?

http://dixdesign.com/radmetal.htm

NO, WE USE STRETCHING WHEELS TO GET THE PLATES LENGTHWISE, OR PUT BANANA in them, then we use forming wheels to finish the shape exactly, If you search my posts you can see a pic of a machine we use You can also see soem of this in my gallery, It is a very satisfying trade, a good man can form 4 plates per day , on a 50 footer there may be 40 plates!!, depends on the degree of complexity
But true some European yards use these reciprecating presses(pounders as you call) to shape, needs two men as aginst one with the plate travelling suspended from a monorail and going through the wheels