X-rays have substantially advanced our understanding of the structure of physical objects, from the macroscopic scale to the atomic scale. In this thesis, we primarily utilize two x-ray diffraction variants—resonant elastic x-ray scattering and non-resonant reciprocal space mapping—to explore the interplay of structure, magnetism, and electronic properties in epitaxial films of RuO$_2$ and Ca$_2$RuO$_4$. We begin with a review of the theoretical principles underlying resonant x-ray diffraction and provide a history of its experimental successes in probing long-range order such as charge density waves, orbital order, and magnetic ordering.We use resonant diffraction to study the structurally forbidden 100 Bragg peak of RuO$_2$, which has been attributed to antiferromagnetic order. We show how epitaxial strain in (101)- and (110)-oriented thin films of RuO$_2$ on TiO$_2$ substrates can tune the line shape of the 100 peak’s $L_2$ resonance profile, with large apparent peak shifts on the order of 1 eV. To explain this behavior, we employ a crystal-field model sensitive to the applied epitaxial strain. After reports that RuO$_2$ is not only antiferromagnetic but also exemplary of a new class of magnetic order called altermagnetism, we show resonant scattering results from a set of epitaxially-strained RuO$_2$/TiO$_2$ (100) films. Using full polarization control, i.e. linear and circular incident x-rays with a downstream polarization analyzer, we demonstrate definitively that the forbidden 100 Bragg peak does not arise from any magnetic order at all; rather, it originates from anisotropic charge scattering visible on resonance. In addition to resonant scattering, another mode of diffraction experiment is reciprocal space mapping, in which the full 3D reciprocal space including dozens to hundreds of Bragg peaks is recorded along with diffuse intensity away from reciprocal lattice points. The shape of the Bragg peaks, the presence of superlattice reflections, and the diffuse intensity connecting them all contain information about the material’s structure beyond the perfect crystal lattice, such as lattice distortions, domain textures, and correlated atomic displacements inside the unit cell. We developed a Python- and Jupyter-based data processing pipeline at the QM$^2$ beamline of the Cornell High Energy Synchrotron Source to collect reciprocal space maps from thin films in mere minutes. We show, as an example, the case of Ca$_2$RuO$_4$, a Mott insulator that forms periodic, ferroelastic nanodomains in real space and butterfly-shaped superlattice Bragg peaks in reciprocal space. We develop an invariant-plane strain model to explain the variations of the Bragg peak shapes at all lattice points. Using our rapid reciprocal space mapping capabilities, we discovered the presence of a crystalline defect phase in RuO$_2$/TiO$_2$ (100) epitaxial films that is strongly correlated with the onset of superconductivity at low temperatures. As a function of film thickness, superconductivity onset begins around 10 nm; T$_c$ increases to a maximum around 42 nm and then begins to decrease; the transition vanishes in films above 50 nm. We show that the presence of planar crystalline defects in these films exhibits the same thickness dependence as the presence of superconductivity, suggesting a possible link.