https://www.gamespot.com/articles/pokemon-lets-go-pikachu-and-eevees-pokemon-go-conn/1100-6461871/

As previously revealed, the upcoming Let's Go, Pikachu and Let's Go, Eevee games for Nintendo Switch will feature connectivity with Pokemon Go. You'll be able to send any Kanto Pokemon caught in the mobile game--including their Alolan variants--over to the Switch titles, where they can then be recaptured at the Go Park, a new facility that replaces the Safari Zone in Fuchsia City. The Pokemon Company has now shared more details on how this process will work.

According to the official Let's Go website, the Go Park is actually comprised of 20 parks, each of which can house 50 Pokemon, meaning you can store up to 1,000 monsters total at the facility. To bring the Pokemon over, you'll need to follow the onscreen prompts at the Go Park's main desk to link your Switch game with Pokemon Go. However, this is a one-way process; any Pokemon you transfer to Let's Go cannot be brought back to Pokemon Go. You also aren't restricted to your own Pokemon Go account; The Pokemon Company says there's "no limit" to the number of accounts you can transfer monsters from.