As Alan Wake, players explore The Dark Place, while Saga explores Bright Falls and its surrounding areas.

Source: https://gamerant.com/alan-wake-2-how-to-save/

Fans, along with Alan, learn much more about Zane's life and personality, and most importantly, his idea about how they both might collaborate on a grand plan to contain the Dark Presence and break out from the Dark Place together for good.

Source: https://gamerant.com/alan-wake-2-thomas-zane-poet-return-story-expansions/

Fiction can change reality in the Dark Place.

Source: https://venturebeat.com/games/why-it-took-remedy-13-years-to-create-the-creepy-detective-tale-of-alan-wake-2-sam-lake-interview/

The game offers two different realities to explore: Bright Falls with its mysterious Cauldron Lake, and a twisted, neo-noir version of New York City called the Dark Place.

Source: https://gamerant.com/alan-wake-2-differences-from-remastered-version/

This grim Faustian catch is Casey becoming the new Scratch, stalking Alan and Saga again through the Dark Place.

Source: https://gamerant.com/alan-wake-2-ending-story-spiral-explained/