When you have collected 100 fragments of one unit, you will be able to summon it by tapping on the unit you choose.

Units that are ready to summon will appear at the top of the list in the Units Fragments tab (ordered by rarity).

The rest of the units are sorted based on the number of fragments you possess for each unit. If you have the same number of Fragments for several units, the appearance order will be: Legendary > Epic > Not discovered.