This paper presents the ETHER architecture, a comprehensive end-to-end (E2E) framework unifying Terrestrial Network (TN) and Non-Terrestrial Network (NTN) infrastructures for future 6G networks. By integrating satellite, aerial, and terrestrial strata into a cohesive3D ecosystem, ETHER addresses the challenges of interoperability, mobility management, and resource orchestration across heterogeneous environments. The proposed layered design builds on key enablers, including NFV, SDN, AI-driven orchestration, flexible payload concepts, and specialised management and orchestration mechanisms to accommodate diverse service requirements. Emphasis is placed on closed-loop automation for dynamic topology management, handover optimisation, and efficient E2E slicing. Additionally, an AI layer extending the Network Intelligence concept is introduced, enhancing situational awareness and performance prediction across the 3D network. The architecture has been evaluated in terms of energy efficiency, demonstrating significant gains for resource allocation under various traffic conditions. Overall, ETHER delivers a flexible, future-proof TN-NTN solution capable of meeting increasingly complex demands in 6G mobile networks.