Superconducting radio-frequency cavities coated with niobium thin films on copper offer an attractive alternative to bulk niobium for future particle accelerators, combining reduced material costs with improved thermal stability. Their wider application is, however, limited by the degradation of radio-frequency performance with increasing accelerating field. The scope of this work is therefore to advance the understanding and optimization of Nb/Cu cavities, with particular focus on their application to the Future Circular Collider. The study investigates how film growth, substrate properties, interface engineering, and cavity processing influence the superconducting and radio-frequency performance of the system. A coordinated experimental program combines thin-film deposition by high-power impulse magnetron sputtering with structural and superconducting characterization and radio-frequency testing. The investigation progresses from small samples to 1.3 GHz cavities and, finally, to 400 MHz cavities, addressing the increasing complexity associated with transferring optimized coating processes to accelerator-scale geometries.