Back in Pokemon Sun & Moon, getting a Zygarde with its Power Construct Ability was quite the task. In order to successfully do so, players had to scour all four islands (as well as Aether Paradise) of The Alola Region in search of a hundred different Zygarde Cells and Cores (after receiving the Zygarde Cube from Dexio and Sina in Heahea City). This was quite difficult to do, as most of these cells were pretty well hidden. To make the search all the more arduous, some cells could only be found during the day while others could only be found at night. It was frustrating in more ways than one, but the end result, a Zygarde in its 100% Form, was well worth it in the end.
However, all of that has changed during your second journey throughout The Alola Region, making it easier than ever before to obtain a Power Construct Zygarde. Now, all you must do is first defeat the Pokemon League. After doing so, you can find and capture Zygarde in Resolution Cave on Poni Island. Once you have captured it, you can head back to the lab on Route 16 on Ula'ula Island to battle Dexio once more. After defeating him, you will receive a Zygarde Cube from Dexio and Sina, one with already 40 Zygarde Cells inside of it. On top of this, you will also receive a Zygarde from the two assistants of Professor Sycamore.
Once you have obtained your second Zygarde and the Zygarde Cube, use the machine inside the lab on Route 16 to separate one of your Zygarde to receive its cells. This unfortunately means you will lose that Zygarde forever, but it will give you just enough cells to use on your other Zygarde to assemble it to have the exclusive Power Construct Ability. Once you have created a Zygarde with said ability, you can see it in its 100% Form during battle once Zygarde has less than 50% of its total Hit Points (without first fainting).