This thesis investigates the significance of relativistic effects in the large scale distribution of galaxies at late times, particularly gravitational redshift, in future cosmic surveys. Traditionally, phenomena like redshift-space distortions and gravitational lensing have been key in understanding the large-scale structure of the Universe and testing General Relativity. Redshift-space distortions highlight peculiar velocities of galaxies, creating anisotropic clustering patterns, while gravitational lensing distorts the apparent positions, shapes and magnitudes of background galaxies due to massive structures bending light in the path towards our detectors. These phenomena provide crucial insights into the matter-geometry relationship in the cosmos. However, additional relativistic effects, often overlooked in cosmological studies, are expected to play a vital role in future analyses. Gravitational redshift, where photons lose energy climbing out of gravitational wells, resulting in an additional redshift, has been measured in astrophysical contexts but not fully integrated into cosmological surveys. This thesis demonstrates that gravitational redshift will be essential for analyzing data from upcoming Stage IV galaxy surveys, serving as a direct probe of spacetime geometry and helping to resolve parameter degeneracies in models beyond General Relativity. Finally, the thesis discusses the potential of relativistic effects to improve the precision of measuring magnification and evolution biases, which are important astrophysical systematics for detecting deviations from Gaussianity or testing the Cosmological Principle.