The new trailer for Pokémon Let’s Go, Pikachu/Eevee details how you can import Pokémon from mobile title Pokémon Go into the Switch title.
Your Pokémon from Pokémon Go appear in the Go Park Complex (made up of 20 Go Parks), and each Go Park can hold 50 Pokémon. Once they are in a Go Park they must then be caught in order for you to use them in the Switch game. When 25 of the same species of Pokémon are in the complex, you can play the minigame (referenced in the trailer) for Candies to power up your Pokémon.
Friends’ Pokémon can also be transferred to a single save file.
Pokémon Let’s Go, Pikachu/Eevee comes out on November 16. For more on the title, check out this previous trailer showing the power of partnerships in the game.
Source: Game Informer How To Put Your Pokémon Go Collection Into Pokémon Let's Go, Pikachu