Polymeric carbon nitrides (PCNs) are intriguing and versatile semiconductors with 2D stacked architecture and hold promise for applications in photocatalysis and optoelectronics. However, our fundamental understanding of their unique electronic and optical properties is still limited, without any clear link between theoretical calculations and the experimentally observed properties. Herein, we investigate the relationship between the structural, electronic and optical properties of PCNs through first-principles calculations. Our results highlight the significant influence of the PCN structure on the electronic and optical properties, especially near the band edges. Specifically, the degree of condensation and corrugation influences the electron/hole localization and the energy levels of $pi$ electrons, which determine the optical behavior. Through the investigation of both 2D and 3D model structures, exciton photophysical processes in PCN materials are elucidated, emphasizing the crucial influence of increasing dimensionality on fundamental optical properties, and establishing a direct link to experimentally observed optical spectra. Further, a mechanism is proposed for carrier and energy transport occurring between the layers, i.e. perpendicular to the material plane. This study provides new theoretical insights into the intricate relationship between the structural, electronic, and optical properties of PCNs, paving the way for a rational design of PCN materials with tuned functionality and improved performance.