set
fun setContainer(holder: PersistentDataHolder, key: NamespacedKey, block: (PersistentDataContainer) -> Unit)
Opens (or creates) a nested container at key, lets block mutate it, then writes it back. The building block for structured storage like setLocation.
fun setContainer(item: ItemStack, key: NamespacedKey, block: (PersistentDataContainer) -> Unit): ItemStack
setContainer for items: opens/creates a nested container at key, mutates it, writes back. Returns the stack.