Minecraft wiki entry for Hopper, associated text: Block entity data
Tags common to all block entities[show]
 CustomName: Optional. The name of this container in JSON text component, which appears in its GUI where the default name ordinarily appears.
 Items: List of items in this container.
: An item, including the slot tag.
Tags common to all items[show]
 Lock: Optional. When not blank, prevents the container from being opened unless the opener is holding an item whose name matches this string.
 LootTable: Optional. Loot table to be used to fill the hopper when it is next opened, or the items are otherwise interacted with. Note that the loot table is used when the hopper tries to push items, when it's enabled.[note 1]
 LootTableSeed: Optional. Seed for generating the loot table. 0 or omitted uses a random seed.[note 1]
 TransferCooldown: Time until the next transfer in game ticks, naturally between 1 and 8 or 0 if there is no transfer.