When you connect your Pokémon Scarlet or Pokémon Violet game to Pokémon GO and receive a Postcard, the pattern of Vivillon that appear in the wild will change depending on the geographic location associated with the Postcard sent from Pokémon GO.
All about Vivillon
Vivillon is a Pokémon that made its first appearance in the Pokémon X and Pokémon Y games. Vivillon had a special characteristic in these games—the patterns on their wings would differ depending on the region settings of the Nintendo 3DS that was being used.
In Pokémon Scarlet and Pokémon Violet, you’ll be able to collect Vivillon of various patterns by trading with Trainers from all over the world who also connected their games to Pokémon GO and by sending Postcards from lots of different countries and regions from Pokémon GO to your own game.
Note: Once 24 real-world hours have passed since you received a Postcard, the Vivillon that appear in your game will once more have the Fancy Pattern (the pattern that usually appears in the Paldea region).