<p>Doped hafnium oxide (HfO₂) ferroelectrics show great potential in next-generation memory and compute-in-memory applications due to their compatibility with advanced silicon-based technology. Typically, HfO₂ shows a reverse size effect, where the polar orthorhombic phase (PO, space group <i>Pca</i>2₁) is stabilized only at thicknesses of a few nanometers. Yttrium-doped hafnium oxide (Y:HfO₂) exhibits a distinct behavior, maintaining robust polarization from ultrathin films to bulk crystals. However, the mechanism enabling Y:HfO₂ ferroelectricity which is critical for expanding device scalability and performance remains unclear. In this work, the multi-field stabilization mechanisms of the PO phase are systematically investigated for bulk and thin film Y:HfO₂ via first-principles calculations. The synergistic effect of composite defects combined with Y dopants and tetra-coordinated oxygen vacancies (Y+V_O4), strain, and electric field significantly broadens the window of thermodynamically metastable PO phase. Notably, we find that the strain requirement can be significantly relaxed with increasing concentration of Y dopants or Y+V_O4 defect pairs, revealing the feasibility of achieving ferroelectricity in bulk Y:HfO₂ without substrate constraints. Moreover, we demonstrate an increase in critical thickness to stabilize the PO phase in Y:HfO₂ thin film compared with pure HfO₂, which is ascribed to the effects of Y+V_O4 defects on the surface energy. These findings clarify the key role of Y+V_O4 defects in realizing thickness-unrestricted ferroelectricity in Y: HfO₂ and provide critical theoretical guidance for optimizing the fabrication process of high-performance HfO₂-based ferroelectric devices.</p>