The development of superior exsolved materials for energy applications necessitates a comprehensive understanding of their atomic-scale structure and the structural modifications that occur both during formation and under operational conditions. A core challenge in characterising exsolved materials is how to extract detailed atomic-resolution information under real-world conditions. This study reports the monitoring of the full nucleation and growth mechanism of exsolved single-metal NPs by in situ thermal scanning transmission electron microscopy (STEM) investigations at the atomic scale. This high-resolution in situ microscopy study allowed to observe atomic diffusion, nucleation sites, the evolution of the host crystal structure, and the role of evolving host defects in the early stages of nucleation, allowing to correlate the defect formation in the host oxide to the nanoparticle formation at the surface during exsolution looking at the process holistically: both atom to nanoparticles nucleation, and perovskite structural evolution at the same scale. Next, to tackle the limited understanding of exsolved materials' exceptional catalytic properties, a combined approach adopting complementary operando characterisation techniques was developed to study the chemical, structural, and microstructural features that determine the catalytic behaviour mechanisms in exsolved fluorite and spinel materials applied to the catalytic process of CO₂ hydrogenation to methanol. By carrying out a complementary in situ-operando study on the behaviour of this category of catalytic materials, a NP-support dynamic synergy was revealed for the exsolved materials, which elucidated their improved performance. By near ambient pressure (NAP)-XPS the evolution of the materials surface composition and reaction intermediate species during catalytic processing was studied, also monitored, together with the identification and role of active sites, by in situ/operando FTIR studies. Through operando STEM, this complementary set of information was coupled with the morphology evolution of the exsolved catalysts, revealing a dynamic faceting and restructuring of the exsolved particles, as well as a reversible surface speciation during reaction, providing insights into the catalytic mechanism of the exsolved materials studied. Using such a combined approach allowed to identify the key factors responsible for the enhanced catalytic activity of the developed exsolved materials while determining the structure-activity-selectivity relationship under dynamic conditions.