<p>The development of spintronic and magnonic technologies hinges on materials capable of supporting pure spin currents with minimal energy loss. Among various candidates, ferrimagnetic insulators are uniquely suited for this purpose, offering high magnetic coherence and low dissipation. This review focuses on the physical mechanisms and material phenomena underlying spin transport in hybrid structures, particularly those incorporating yttrium iron garnet (YIG). Central spintronic effects, including spin Seebeck and spin Hall effects, spin pumping, magnetic proximity effects, and spin, orbit, or spin transfer torques, are examined in detail, emphasizing their dependence on interfacial structure, chemical composition, and magnetic hybridization. Advances in nanofabrication of high-quality YIG thin films and the engineering of spin–orbit-active interfaces are discussed in the context of energy-efficient spin logic and coherent spin wave-based devices. By bridging fundamental insights with applied developments, this review aims to outline the state of the art and identify key challenges and opportunities in insulating spintronics.</p> Graphical Abstract <p></p>