From a search on a few sites they sell them whole to metal recyclers, apparently they use a few work arounds to thwart recording keeping.
Not one said they cut them open (my guess you don't what to breath those metals)
Well, you can believe Google, or you can believe a retired cop that has actually investigated plenty of these cases.
Individual cats are vehicle-specific and stamped with part numbers that increase the risk of someone like me being able to match up a particular cat to a particular theft case. So they cut them open with an angle grinder, remove the "bricks" from inside, ditch the shell, and then take the bricks to the metal recycling place and sell them for the platinum, palladium, and rhodium for meth money.
Most theft recoveries of this type happen as a by-product of another type of contact- for example if I see a tweaker-mobile cruising a residential neighborhood at three in the morning, often a known tweaker and/or a known tweaker-mobile that I know doesn't belong to that neighborhood, I'll stop them for any legally justifiable reason (license plate light out, touched a painted stripe, expired tags, etc.), and then when I walk up, I'll see a load of stolen stuff in the back. From there, I call my beat partners, and we all get on our computers and start trying to match the stolen property up with specific cases...