This work presents a unified derivation of the complete structure of the Milky Way within the framework of the Boundary Interface Theory (BIT). Without introducing dark matter, dark energy, or ad-hoc assumptions, we systematically explain the galactic rotation curve, thin/thick disk, spiral arms, central bar, nuclear bulge ellipticity, disk warping, and halo density distribution. All derivations are mathematically self-consistent and dimensionally verified, and the theoretical predictions are consistent with high-precision astronomical observations. This paper is a direct application and extension of the Boundary Interface Theory, demonstrating its ability to describe galactic-scale structures in a unified and parsimonious manner.