It has to already have all the data in it; it’s not like Ash (or Red/Blue/etc) actually wrote anything in it. It just doesn’t let you access the data until you’re near or have captured a pokemon it had data on. Because it also doesn’t have data on some of the unique pokemon from what I recall from one of the movies. Tho it does have data on Mewtwo in the game, and that mofo was made in a supposedly top secret lab. Oak must have been involved in his creation… 🤔