I wont say who I am, but I have some pertinent information regarding the new releases. Up to you whether or not you believe, I am just posting this here because I would like to see the reaction when this is eventually revealed.

The island that you saw in the trailer is the only island in the base game. It consists of your house, your friends' houses (you have two 'close' friends'), some neighbour houses and the school campus which is essentially the main hub.

The game is based on completing objectives given to you by the school as 'assignments'. This is the central focus of the game. The first assignment is to catch a Rattata (serving as a kind of tutorial). The second assignment is to compete in a battle against a fellow student to show how battling works (you pass the assignment regardless of whether you win or lose). The third assignment is to then catch 5 Sewaddle, and so on - the assignments get progressively more bigger/longer as the game progresses in the same fashion.

The catching and battling mechanics are the same is in Pokemon Legends: Arceus - they are essentially cut and pasted over (what I saw used exactly the same fonts etc. although that is probably placeholder at this time).

There is some murmuring about possible DLC islands post launch. However this is not concrete yet so not confirmed or definite.