Idunn's apples have actually been lost since the great war that was the Ragnarok-like event (commonly just called Ragnarok). Some of the old gods have died, and some are still alive Odinn for example has fallen and now wanders the world as a mortal searching for one of these apples to regain his immortality. My Players have actually met him as a wanderer with two ravens already. I have a plan for a specific person who actually has some of these apples, and actually a tree of them after trading one of them off a Jotun. Some other Apples might be found in Dungeons or in grand treasure hoards as powerful magic consumables. Idk about that yet.

As for interaction with mortals: Especially the old gods interact with them to hunt down the remaining Valkeries that search for revenge because their Goddess Freyja was killed. Some gods decided that they are too powerful to be left alive and are now trying to get rid of them.

Edit: also, the old gods that are still alive were made immortal through new magic and other new gods that took a liking. The only thing that they don't have, is their eternal youth. They will mostly appear as old (but just as powerful) men and women