How does gravity work under the hood in the physics engine?

When you set the world gravity, what exactly happens under the hood?

If I wanted to replace the built in gravity with my own, and have the behavior look identical to Unity's gravity, would the code be this simple, or is there more going on each tick?

 void FixedUpdate() {
     const float gForce = -9.81;
     Vector3 gravityVector = new Vector3(
         0.0f,
         gForce * Time.deltaTime,
         0.0f
     );
 
     rigidBody.AddForce(gravityVector);
 }