Does the aircraft storage site in the desert, only destroy old aircraft? Or do they moth ball aircraft for use in a future mobilization ?
Yes >:D
Both, they mothball them for a time if the airframe is still in active service and the aircraft is usable. They also strip them for parts and send them to the active fleet if needed. Go on Google Earth and search for Davis-Monthan AFB and you can see rows and rows of aircraft in various states of preservation and destruction.
There are numerous desert storage locations (deserts make pretty good aircraft storage environments, so yeah), but I'm going to assume that you mean the Aerospace Maintenance and Regeneration Center also known as The Boneyard at Davis-Monthan Air Force Base in Tucson, Arizona. You can read more about it here (https://en.wikipedia.org/wiki/309th_Aerospace_Maintenance_and_Regeneration_Group). In short, the recycle and reclaim aircraft, refit aircraft for sale to friendly governments, and scavenge aircraft to support airframes still in service.
Many years ago, back when my dad was a kid living in Tucson, you could simply walk right into the boneyard. Not so much today.