Legendary and Ultimate items do not drop directly from outfit chests.
Instead, chests give blueprints for these items.

A blueprint shows the pieces you need to craft the final item.
Each blueprint requires specific Rare and Epic items.
Once you collect all required pieces, you can craft the Legendary or Ultimate item.

To see which pieces you need, tap the item card and open Set info.
This will show the full blueprint and the items you still need to collect.

You can craft high-rarity items using the blueprints and required lower-rarity pieces.

You can also upgrade items by using equipment you no longer need.
Each item has a value that counts toward the upgrade:

Rare item = 2 Common

Epic item or Legendary blueprint = 4 Common

Legendary item or Ultimate blueprint = 8 Common

Ultimate item = 16 Common

If an item has already been upgraded before, its value will be higher.

Keep in mind that items used for crafting or upgrading are consumed and disappear after the process.