In Pokémon Pokopia, the structure that launches into outer space after the final celebration is a rocket ship.
The story initially presents the mysterious structure as a tower connected to the Team Initiation Challenge. Ditto must provide requested items for the supposed wellbeing of Pokémon, while Professor Tangrowth helps investigate the abandoned world and its unexplained history.
After Ditto and Professor Tangrowth restore habitats and complete the challenge, they celebrate with party poppers. That seemingly harmless event activates an emergency evacuation protocol, revealing that the structure is actually a rocket ship. It then launches into outer space with items connected to the game's story.
This revelation helps explain the game's post-apocalyptic setting. Human records indicate that people left the planet for space after environmental changes made the landscape hostile to life. They could not take every Pokémon with them, so a computer system was created to preserve them and release some if it began to fail.