https://www.polygon.com/guides/2018/11/26/18106715/poke-ball-plus-pokemon-lets-go-iphone-android-connect-buttons-lights-bluetooth-troubleshoot

Nintendo took advantage of Pokémon: Let’s Go! to release a new peripheral, the Poké Ball Plus, which works both with the Switch game and mobile game Pokémon Go. It’s more than just an accessory to connect Let’s Go! to Pokémon Go on mobile; it’s also a controller for Let’s Go!.

We’ve covered the transfer part of the process, which allows for sending Pokémon from the smartphone to the Switch. For this guide, we’re going to focus on the other functions of the Poké Ball Plus: how to play Let’s Go! with it on Switch, and how to use it to collect Pokémon, items and Candy on phones.