RF/mmWave systems require 3D packaging solutions that maintain signal integrity while enabling heterogeneous integration, yet conventional implementations remain constrained by lossy off-chip interconnects and package-bound architectures. This thesis investigates whether interconnects can be engineered as independent building blocks for heterogeneous RF/mmWave systems. It presents a 3D stitch-chip-based, interconnect-focused integration platform that decouples interconnect design from active dice and enables independent optimization of electrical performance and integration flexibility. The concept is established through 3D stitch-chip fabrication and extended to active-die integration. Stacked-via and glass-via implementations further improve vertical scalability and broaden the architectural scope of stitch-chip-based integration. To address the isolation and loss limitations of conventional TSVs for RF/mmWave vertical integration, airgap-isolated semi-coaxial TSV structures are developed to reduce parasitic effects and enable 50 Ω design at reduced pitch. Finally, a liquid-cooled pin-fin platform incorporating airgap-isolated TSVs demonstrates the feasibility of extending the proposed interconnect approach toward thermally managed multi-tier RF/mmWave integration.