I'm wondering - how do you make an item that uses a different model when held?

The vanilla trident and spyglass items both have separate models: "trident_in_hand.json" and "spyglass_in_hand.json". I would like to know how this is done to add this feature to a new item.

Thanks!

Those are technically two separate systems, but you can use a Forge model loader for it. That will change models depending on the specified perspective.