The significant quest associated with the Fingerstone area in Elden Ring's Shadow of the Erdtree DLC is the Finger Ruins questline given by Count Ymir, located in the Cathedral of Manus Metyr1. This quest involves visiting various Finger Ruins locations, interacting with mysterious bells, and ultimately leading to a unique boss fight against Metyr, the Mother of Fingers.
The Hinterlands is located in the northeastern part of the game map in Elden Ring's Shadow of the Erdtree DLC. It is a secret area that requires the "O Mother" emote to access, which can be obtained near Bonny Village. Once the emote is used in front of a statue in the Shadow Keep, Back Gate, a hidden door leading to the Hinterlands is revealed2.
The O Mother emote is crucial for accessing the Hinterlands in Elden Ring's Shadow of the Erdtree DLC because it unlocks a hidden path. By performing the emote in front of a specific statue in the Shadow Keep, the statue moves aside, revealing a door that leads to the Hinterlands, an area containing valuable rewards and a significant quest1. Without the O Mother emote, players would not be able to access this region.