Poipole's homeworld is an alternative dimension located in Ultra Space in the anime. It is a counterpart to Ultra Megalopolis.
History
Originally Poipole’s homeworld was filled with flowers thanks to the presences of Ultra Necrozma, Solgaleo and Lunala. By the time after Ultra Necrozma prevents the meteorite fall into this dimension with impact, at cost of losing its light upon returning to into a regular Necrozma and went into sealed state for years. Without the said Ultra Beast Pokémon and its light along with the absences of Solgaleo and Lunala, this dimension turned into a desert and shrouded by darkness for few years later, leaving the pack of Poipole endangered and even the pack’s leader Naganadel becomes weakened and old.
Thankfully in many years later, one of the Poipole who would eventually be under care of Ash manage to find an Ultra Wormhole and get a help from one of Ash’s teammates within Ultra Guardians as well.
At time after the awaken of Necrozma while in its fallen form due to the presences of Nebby and Lunala, just as the Ultra Guardians chases them through Ultra Wormhole, they finally found this world where Ash’s Poipole lost from and being the place where Necrozma was sealed in. Once the Ultra Guardians learn the history behind Necrozma’s sudden downfall from Naganadel, they proceed to the new plan to transfer the Z-Crystal powers to Necrozma without losing both Solgaleo and Lunala, with the help from the civilians in Alola including Team Rocket Trio, followed by the pack of Poipole, and finally the Solgaleo and Lunala’s Z-Moves by Ash and Gladion. With Necrozma finally restored to his heroic Ultra form at the time after the Solar Eclipse occur, both Alola and Poipole’s world are restored at the same time. Now that its world has been restored, the Poipole which was belonged to Ash bids its friends a farewell and will meet again in another time.