Just like how you can send Pokemon that you have captured in Pokemon GO to Pokemon: Let's Go, Pikachu, you can also send a few things from Pokemon: Let's Go, Pikachu to Pokemon GO. The most important thing one can send to Pokemon GO is the Pokemon known as Meltan, since a player can only evolve Meltan into Melmetal within Pokemon GO. This connectivity is required in order to get both Meltan and Melmetal in Pokemon: Let's Go, Pikachu.
On top of being able to send Meltan from Pokemon: Let's Go, Pikachu to Pokemon GO, players also have the ability to send a special Mystery Box to the hit mobile game. In order to do this, all one must do is connect the two games. Once this has been done, the Mystery Box will automatically be sent to Pokemon GO. After opening the box in Pokemon GO, players will have a thirty minute window to capture additional Meltan in Pokemon GO. After exactly one week, this same Mystery Box can be reopened. In order to reopen it, however, you will need to transfer more Pokemon to Pokemon: Let's Go, Pikachu.
Need some help in Pokemon GO? Check out our Pokemon GO Guide!