onurcangoc Thanks for asking the question with the video. The video helped me to quickly identify the cause of your problem. 😃
The reason this happens is because the hierarchy of the bones is not appropriate:
Normally transforms from parent bones affect their child bones, so if the parent bone rotates, the child bone will also rotate. So if you make the bone for each finger parent the bone for the palm, it will not be affected by the rotation of another finger's bone.
A bone's parent can be changed by clicking Set Parent or pressing P, then clicking the new parent bone in the tree or viewport. A bone can also be dragged in the tree to a new parent bone, even in a different skeleton.
