https://nintendoeverything.com/more-pokemon-lets-go-pikachu-eevee-details/

– You will not be able to evolve your partner Pokemon
– However, can catch other Eevee/Pikachus you find in the wild
– Your partner Pokemon will take up one of your six party slots, and you will be able to box it
– Boxes work differently in that you now have a box in your bag
– The ability to transfer from Go/Let’s Go opens up about halfway through the game
– After that, you’ll be able to do it freely
– There are also minigames you can do when transferring lots of Pokemon over
– Candy from Go will not do anything in Let’s Go
– Won’t say whether you’ll be able to transfer Pokemon from Let’s Go and turn them into Candy
– Any Pokemon that’s large enough in Let’s Go you will automatically ride
– They had the idea to let you ride Pokemon of any size but decided against it
– No HMs
– Although the interface is different, you will still earn EXP from catching Pokemon, as well as trainer battles
– Event Pokemon in Go (Ash hat Pikachu, etc), cannot be transferred to Let’s Go
– The controls for catching Pokemon in handheld mode will have you moving the console around and pressing a button to throw; no touch screen, and the gyro part is required