Two ways to get to a game's Pokedex: through the Games page, or via the URL pattern /pokemon-[game]-pokedex. Both go to the same page.
Step 1
Easiest: click the game's cover on the Games page.
Step 2
Direct URL: /pokemon-red-pokedex, /pokemon-scarlet-pokedex, /pokemon-sword-pokedex, and so on. Replace [game] with the game slug.
Step 3
From any Pokemon's detail page, click a game in the Appearances sidebar on the right. That takes you to the Pokemon's page for that specific game, and from there to the game Pokedex.
Step 4
Region Pokedexes use a different URL: /pokemon-kanto-pokedex, /pokemon-johto-pokedex, etc. Those cover everything in a region across multiple games.