As we're all painfully aware by now, Pokémon Sword and Shield does not include a complete National Pokédex, and a great many veteran 'mon have been left out of the game. However, where there's a will, there's a way, and modders are already hard at work filling in the gaps where they can.
For the example shown, Yamper's entry was edited using the save editor PKHeX, but Omastar hasn't supplanted Yamper – instead, it has been given its own unique ID. The catch is that Omastar lacks some key animations that are unique to the game, such as Dynamaxing.
Another problem is filling in monsters which weren't present in the Let's Go games – 3DS models could potentially be used, but it's not yet known if it's possible. However, SciresM appears to suggest that the job isn't one he's willing to take on, and that the whole process is simply to show that it's possible to insert new models and animations into Sword and Shield.
Game Freak has already stated that a complete Pokédex won't be coming to the game, and that future sequels may also ship with an incomplete Pokédex.