6G is expected to trigger a total rethink of network architecture design, adopting the new Network of Networks vision which promotes the integration of multi-stakeholder, multi-technology infrastructures. To this end, future 6G systems will tightly integrate upcoming satellite systems and Low-Earth Orbit constellations (e.g., OneWeb) as well as High-Altitude Platforms (HAPs) and unmanned aerial vehicles (UAVs), a concept termed as integrated Terrestrial and Non-terrestrial Networking (TNTN). However, the sheer scale and heterogeneity of future TNTN infrastructures is expected to create a unique set of challenges. To this end, 6G-TERRAIN adopts Integrated Communication and Sensing (ICAS) as a key enabler of TNTNs, allowing to perceive the complex and dynamic TNTNs through sensing the physical environment. Moreover, 6G-TERRAIN aims to propose, design, and validate a novel Native AI automation architecture and its associated enabling technologies and theoretical frameworks, aiming for an intent 6G System with seamlessly integrated multi-stakeholder & multi-technology (e.g., terrestrial and non-terrestrial) nodes. Natural Language intents are adopted, exploiting the breakthrough capabilities of emerging LLMs to automatically plan the optimal decompositions of intents into tasks and prioritise their downstream execution. Finally, 6G-TERRAIN will build a training network to conduct top-notch research towards the development and experimental evaluation of a gamut of techniques, methodological frameworks and tools that will break the barriers currently impeding the realisation of the 6G “Network of Networks” vision.