Efficient spin transfer in the van der Waals heterostructure <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>WTe</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:mo>/</mml:mo> <mml:mrow> <mml:msub> <mml:mi>Fe</mml:mi> <mml:mn

Recent experiments have demonstrated efficient spin transfer across layers in the van der Waals heterostructure composed of ${\mathrm{WTe}}_{2}$ and ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$, signaling a potential breakthrough in developing all--van der Waals spin-orbit torque devices. However, the reasons behind the unusually high interlayer spin transparency observed, despite the weak van der Waals interactions between layers, remain unclear. In this study, we employ density functional theory and the nonequilibrium Green's function method to explore this phenomenon. We find that the efficient cross-layer spin transfer arises from direct hybridization of $p$-orbitals between tellurium atoms at the interface. This interlayer orbital hybridization lowers the electronic potential barrier and significantly modifies the spin-polarized electronic structure of ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. Consequently, an effective channel for spin-polarized transport is established between ${\mathrm{WTe}}_{2}$ and ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$, leading to high interlayer spin transparency. Combining this enhanced spin transparency with the large spin Hall angle of ${\mathrm{WTe}}_{2}$ explains the high spin-orbit torque efficiency observed experimentally. Furthermore, we predict that applying a gate voltage can further increase this efficiency. Our findings offer a pathway for designing high-performance, all--van der Waals spin-orbit torque devices.