Noob question: How does GetBoneTransform know which transform is which bone?

I am trying to understand some code in a tutorial project that is calling GetBoneTransform. I see it is called by the Animator. When I click on the Animator in the editor, there is no apparent way for Unity’s code to know anything about which of its child transforms are which bone. Yet, it somehow knows to pick the relevant transform with ConfigurableJoint e.g. the one called Arm.L if asked for left arm bone.