In Where Winds Meet, the Longevity Fruit and the Longevity Fruit Seed are two important items. One you receive as part of a quest reward while other you will have to search on your own. Here are the locations for both.

Longevity Fruit Seed: Location

The Longevity Fruit Seed is an item you receive upon defeating the boss the Void King in the Gleaming Abyss Campaign Challenge.

To reach this point, you will have to begin the quest. Then venture into the Gleaming Abyss dungeon.

Defeat the Void King boss at the end of this dungeon to collect the Longevity Fruit Seed.

Longevity Fruit: Location

Use the Fast Travel feature to get to Witherwilds. Use the Wraith Village teleport waypoint.

From your spawn point, go to the house on the left.

Climb the ladder.

You will find the Longevity fruit on the floor before a cupboard.

And that's it. You now have the Longevity Fruit as well. Its description reads:

A glowing fruit, but it seems inedible.
Many years ago, a longevity fruit appeared near Qinghe. Unfortunately, it was short-lived, and no one knows how it grants eternal life.

Between the two, the Longevity Seed is arguably harder to because it involves a sprawling quest with a boss fight at the end.