Integrating two-dimensional (2D) hexagonal boron nitride (h-BN) with three-dimensional (3D) semiconductor materials offers promising opportunities for multifunctional integrated device architectures. However, direct heteroepitaxy between materials of different dimensionalities remains challenging. Here, the polarity-dependent growth behavior of h-BN films grown by metalorganic vapor phase epitaxy (MOVPE) on AlN/sapphire substrate is reported. The surface polarity of the underlying AlN strongly guides the growth mode of h-BN. On N-polar AlN, h-BN nucleates and grows via van der Waals epitaxy (vdWE), yielding well-oriented 2D films. In contrast, Al-polar AlN promotes interfacial Al–N bond formation, leading to 3D island-type BN growth and a rough morphology. First-principles calculations reveal that N-terminated surfaces spatially and energetically suppress interfacial bonding, whereas Al-terminated surfaces favor strong covalent interactions and significantly larger interfacial binding energies. The crystalline quality of wafer-scale h-BN was further enhanced by high-temperature face-to-face annealing (FFA), achieving a Raman E_2g full width at half maximum (FWHM) of 18.3 cm⁻¹. Additionally, composite FFA (CFFA) enabled the fabrication of peelable, flexible AlN films facilitated by the weakly coupled h-BN interface. These results clarify the polarity-driven epitaxial mechanisms of BN on AlN and provide a viable pathway toward scalable 2D–3D hetero-integration and flexible-based nitride electronic and optoelectronic devices.