Our multi-world system lets us determine which worlds are connected and which worlds are completely separate. On Blocklandia only Peaceful, Mainland and the Nether are connected. This means your inventory should travel across these worlds.
However if you enter Darkness or Creative your inventory is completely separated. This means anything you find or make in Darkness stays in Darkness, and the same goes for Creative. This also means you can’t find something in Peaceful and take it into Creative or Darkness.
Pets are also unable to travel between worlds.