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