For players trying to add all of the returning Pokémon that appear in Sword and Shield’s Isle of Armor expansion, Porygon might be a little confusing to find at first.
You won’t be finding the Virtual Pokémon hanging out in the Wild Area, but you will receive it as a gift after finishing the main story of the Isle of Armor.
Once you finish training your Urshifu, complete the quest to unlock its Gigantimax form, and defeat Master Mustard in one last battle at the Master Dojo, head into the lounge inside the dojo — the room to the right as you enter.
Mustard will be relaxing in the room, playing something on the Nintendo Switch and a Porygon will be floating on the right side of the room. All you need to do is walk up to the Porygon and interact with it, which will prompt you to answer if you like the creature to Hyde.
If you say yes, Hyde will explain some details about Porygon, including that you will need to find an Upgrade—the item needed to evolve Porygon into Porygon 2 through trade. After listening to his rambling, Hyde will offer you the Porygon and you can add the Normal-type to your team.
This is the only Porygon known to be available in Sword and Shield, so make sure you save if you want to try and get a shiny through soft-resetting. Otherwise, you can add it to your team and evolve it at your leisure.