The function called HumanoidAvatar is being used when animating human shaped 3D models in Unity.
This page explains the structure of the HumanoidAvatar.

• Unity Tutorial Humanoid Avatars
• Unity Avatar Mapping tab
• Unity Official Blog - Mecanim Humanoid

In Unity, the bone structure for Humanoids (characters) are standardized for the efficiency of productions.
The Humanoid bone structure designated by Unity is called UnityHumanoidAvatar.

Unity doesn't control the bones directly, instead, it remaps the bones of a 3D character to UnityHumanoidAvatar and the result of the HumanoidAvatar being controlled is applied to the character's bones to animate the character.

Arms and Legs are separated into right / left.

Finger joints are named from inner to outer …[Thumb Proximal] > [Thumb Intermediate] > [Thumb Distal].