Many mysteries still hover around the birth of monsters and the process of their manufacture remains to be revealed. Since the monsters roam in a virtual world, the existence of a factory is highly unlikely. It is possible that they are created from a basic program, with all their characteristics, allowing X.A.N.A. to create many monsters, all almost identical. This would correspond to the program created by Jeremie to virtualize vehicles. In some episodes, such as Marabounta, X.A.N.A. virtualizes creatures directly in the field as Jeremie does with the Lyoko Warriors. The Mantas provide another part of the answer: when the heroes first access the interface to Carthage, Mantas appear as if they are 'hatching' from the walls of Sector 5 - they seem to spring from within the digital data. If they are created from a program, in any case, they are not virtualized (within Sector 5 at least).
It is also unclear whether or not X.A.N.A. needs energy to create its monsters. X.A.N.A. produces many of them, but it remains impossible to learn whether large quantities can be produced simply, or whether the quantity of monsters can be infinite; in which case the question remains to be at what rate can X.A.N.A. produce them? It is difficult to gauge from the show as in some episodes the heroes face three monsters, and in others they are forced to fight entire armies. It is certain, however, that X.A.N.A. can produce more monsters as it gains in power with the returns in the past. The appearance of the Kolossus proves this, since it can only be mobilised by X.A.N.A. with multiple Replikas as sources of energy.
The death of the monsters is less convoluted; when they are mortally wounded or fall into the digital sea, they simply devirtualize. They do not appear to have a conscience of any kind, so they do not have any attachment to Lyoko as the heroes would. The destruction of their virtual envelope corresponds to their complete disappearance within the virtual world.
Monsters on Lyoko