Whats the source code of Quaternion.FromToRotation?

How is that relevant? That's a pure mathematical thing. It's probably just something along the lines:

 public static Quaternion FromToRotation(Vector3 aFrom, Vector3 aTo)
 {
     Vector3 axis = Vector3.Cross(aFrom, aTo);
     float angle = Vector3.Angle(aFrom, aTo);
     return Quaternion.AngleAxis(angle, axis.normalized);
 }

Of course AngleAxis should be pretty straight forward as well

 public static Quaternion AngleAxis(float aAngle, Vector3 aAxis)
 {
     aAxis.Normalize();
     float rad = aAngle * Mathf.Deg2Rad * 0.5f;
     aAxis *= Mathf.Sin(rad);
     return new Quaternion(aAxis.x, aAxis.y, aAxis.z, Mathf.Cos(rad));
 }

All untested.