Here is an overview of the demo, via Nintendo.com:
Download the game demo for free!
Hello new Pokemon Trainers, a demo of Pokemon: Let’s Go, Pikachu! and Pokemon: Let’s Go, Eevee! games is available just for you. Explore Viridian Forest, where you can catch Pokemon in the wild and battle trainers with either a partner Pikachu or a partner Eevee at your side (randomly selected). Start your journey now.
* Final game content may vary from this demo.
* Demo game data cannot be transferred to the purchased game.
Pokemon: Let’s Go, Pikachu! and Pokemon: Let’s Go, Eevee! launched worldwide for Switch on November 16, 2018.