As the name of the game might suggest,
Pokemon: Let's Go, Eevee does have connectivity with
Pokemon GO. With it, trainers can send their Pokemon from
Pokemon GO to
Pokemon: Let's Go, Eevee, allowing trainers to capture said Pokemon within the latter game in an area to be called GO
Park. In GO Park, players playing
Pokemon: Let's Go, Eevee can interact and capture the Pokemon they sent from
Pokemon GO.
Unfortunately, only the Pokemon from
Bulbasaur to
Mewtwo (all Kanto Region Pokemon outside of
Mew) can be sent from
Pokemon GO to
Pokemon: Let's Go, Eevee, including the various
Alola Region Forms for the
1st Generation Pokemon that have one. Furthermore, Pokemon sent from
Pokemon GO to
Pokemon: Let's Go, Eevee cannot be returned once sent to GO
Park. (But you will receive
Candy in
Pokemon GO for sending your Pokemon to
Pokemon: Let's Go, Eevee.)