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Well, seeing as you need to have Regigigas in your party to find them and that's a post-game mission reward, you can just Mystery Gift a Gen 8 'mon in to your game to unlock catching other Gen 8 'mon.
It functions much like the DexNav in ORAS; basically, the higher the level, the more info you learn about the Pokémon in the shaking grass/dust cloud/bubbling water. At Level 0, you just know that it is the species you're looking for. As you continue scanning that pokemon, the level increases...