https://www.gamespot.com/articles/heres-how-to-get-meltan-in-pokemon-go/1100-6462405/

While Pokemon: Let's Go, Pikachu and Let's Go, Eevee pare back the series' sprawling bestiary to only the original 151 monsters (plus their Alolan variants), players will be able to obtain one entirely new Pokemon in the games: the strange Mythical Pokemon Meltan. The Pokemon Company had previously teased that the way to acquire Meltan involved linking Let's Go with Pokemon Go, but now we know exactly how you can get your hands on one.

According to The Pokemon Company, the first step to catching Meltan is to send a Pokemon from the mobile game over to the Switch titles. After transferring a Gen 1 monster from Pokemon Go to Let's Go, you'll receive a special item called a Mystery Box in Pokemon Go. Unlike most other items you can get in the game, the Mystery Box is reusable; you'll be able to activate it every so often, which in turn will give you a window of opportunity to capture Meltan in Pokemon Go. You'll then be able to transfer the Mythical Pokemon over to the Let's Go titles and add it to your collection there.