There are a number of Pokemon with branching evolution trees, in which various factors can determine if a Pokemon evolves into one form or another. Gloom, for example, can either evolve into Vileplume via a Leaf Stone or Bellossom via a Sun Stone. Pokemon Sword and Shield introduce a new Pokemon with a branching evolution line, but uniquely, it evolves differently based on which version you're playing.

That's only sort of correct, though. Sword and Shield each have an exclusive evolutionary item, and each one can be used to get a different evolution of the grass/dragon Pokemon Applin. In Sword, you can get a Tart Apple to evolve Applin into Flapple, and in Shield, you can use a Sweet Apple to evolve it into Appletun. As the names would imply, this family is all apple-themed. All you have to do is use the Apple on an Applin like an evolutionary stone.

There's a boy on the west side of Hammerlocke who will clue you in to how to evolve Applin, and he'll even give you the respective Apple item. You can also find either a Tart or Sweet Apple under a tree on Axew's Eye in the Wild Area, and they're also one of the many sweets-related rewards for victory at the Battle Cafes in Motostoke, Hammerlocke, and Wyndon. While we haven't been able to test it yet, we assume that you can give an Apple to any other Pokemon to trade with a friend and get the other version's item in your game so you can evolve your Applin as you please. We'll update this story when we can confirm if it works.