>>34157230All of them.
There are many different worlds, all existing in ultra space. Think of each world as a little pocket dimension. Gens 1-5 happen in one world. Gens 6 and 7 along with the gen 3 remakes happened in a world with Megas. USUM happens in a 3rd world that is undergoing an invasion by necrozma.
Headcanon:
Ultra space is a finite area within space
Each world is a pocket dimension with an identical starting point (maybe something happened in world zero to create infinite, slightly duplicated versions of itself?)
The closer to the center, the more intense the radiation.
World one exists near the edge of Ultra Space.
Mega Worlds and worlds undergoing a Necrozma invasion exist near the core.
Ultra Beast worlds exist extremely close to the core.
There are many Necrozma and many Zygarde. A world with a fully assembled, fully powered Zygarde, or possibly one with multiple ones as seen in in the first Mega World can defeat and bury Necrozma (necrozma was "asleep underground) in order to force it into it's prism form. After this, Zygarde gradually breaks down, the cores and cells spread out over the world and gathering at the point of a potential threat
Kalos therefore recently was the site of a Necrozma invasion and the 50% form you find is in the process of breaking down.
USUM world is even closer to the core and only has a single Zygarde.
