In high-power electronic devices, the thermal conductivity of polymer composites is governed not simply by the intrinsically high thermal conductivity of the fillers, but by their ability to form continuous heat conduction pathways with few structural defects within the polymer matrix. This review examines the selection of carbon-based, ceramic, and metallic fillers, and discusses how hybrid filler systems, filler loading, and surface modification influence filler dispersion, interparticle distance, interfacial contact, and network formation. Vacuum-assisted filtration, template-assisted methods, electrospinning, field-assisted alignment, and three-dimensional printing are further considered in terms of their effects on filler arrangement and connectivity. Filler size, morphology, surface chemistry, and field responsiveness guide fabrication method selection. Fabrication then determines thermal network continuity. Future research needs to develop efficient thermal pathways at reduced filler content and porosity using stable, scalable processes.