When you get an Ultimate part for the first time, one part appears in the inventory. All subsequent copies are added to this part's progress level bar as 1/1 or 2/5, for example. As soon as the bar is full, the Upgrade button becomes active.
Once the necessary number of identical parts are collected, you can improve this part with a special currency (purple banknote with a star). To improve the part you need to tap on it and then tap Upgrade.