I think that it begins in the vestibular system, and radiates out from there.
“In part of this system, we have a gyro stabilized reference platform (the vestibular system) that is directly mapped to the eye tracking system to keep the eyes from being distracted by self-motion. (At that point, in that little spot on the brain stem, there is the closest thing that if you had the correct instrumentation, you could measure as the neural correlates to a sense of self.”
To help those joining the conversation, this thread walks though this basic chain of connections with the second post covering the details of expression in the posture system.
We learn to extend our control out from this frame of reference with the ability to project our agency “into” attached systems. Watch a skilled gamer or a backhoe operator to see this extension of agency in action. For that matter, typing or playing an instrument can reach the point where it is an extension of your body.