The Pokemon Company has commented on the structure of Pokemon Legends: Arceus, and confirms that the game won't be open world.
nintendoeverything.com
So according to this, the world and its areas aren’t gonna be seamless transitions into each other. It’s gonna be less Breath of the Wild and more Monster Hunter where you depart from your base and do things in a big area. I’m okay with that as long as I can leave town to goof off in the world whenever I want, coming and going as I please. I don’t think there was anything in the footage we saw that contradicts that possibility. Like, I can’t imagine you leaving town for a specific mission and only be allowed to do things pertaining to that mission. I’m interested to see some gameplay for these expedition mechanics.