Pokemon Legends: Arceus isn't set in modern times and isn't set in a time period with advanced technology. In modern games, Pokemon are stored using a robust computer system. In Arceus, though, you are going to have to leave your extra Pokemon at a large pasture.
The pasture is located in Jubilife City and is marked on your map. You can talk to the lady standing in front of the gates to leave Pokemon in the pasture or take them out. This system works the same as the PC in the other Pokemon games, so it should be familiar.
Related: How to evolve Pokémon with friendship in Pokémon Legends: Arceus
While you are out in the field, you can speak with the NPC at camping areas to access your pasture. Here you can easily swap out your Pokemon for ones that you want to place on your team or want to focus on leveling for help bringing up your Galaxy rank.
You can also release Pokemon here if you need to and check their stats before adding them to your team.
To learn more about Pokemon Legends Arceus, check out How to check Pokédex registration conditions in Pokémon Legends: Arceus on Pro Game Guides!
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