The most notable bit of connectivity goes from Pokémon GO to Pokémon Let's Go. With this, you can send Pokémon from the Kanto Pokédex that you caught in Pokémon GO into a special area in Let's Go known as "GO Park". This area has you interact with the various Pokémon on the overworld and then capture them.
Only Pokémon from Bulbasaur to Mewtwo can be sent through this, including the Alola Forms of Pokémon with Alola Forms. Mew cannot be transferred.
This capture is only one way. Once you have caught it in the GO Park, it cannot return to Pokémon GO. When you transfer to Pokémon Let's Go, the Pokémon GO game gets some candy