The exploration of matter structures and phase transitions under extreme conditions stands as a pivotal frontier in atomic/molecular physics and high-energy density physics. It holds immense significance across diverse domains, including inertial confinement fusion, celestial evolution, weapon damage effects, and the development of national defense technology. Extreme conditions, like those found in laser and shock experiments, give rise to matter states characterized by elevated temperatures, intense pressures, and highly ionized states of matter. Elucidating the associated behaviors necessitates the creation of multiscale simulation methodologies, spanning from electronic structures to interatomic potentials and even larger scale. In this presentation, we introduce a multiscale model capable of efficiently addressing electrons, ions, and their interconnected modes, and an AI-empowered simulation method is illustrated to extend calculations with ab initio precision to 1 million atoms. These methodologies aim to comprehensively elucidate the dynamic evolution of atomic structures when subjected to extreme conditions. Numerous groundbreaking phenomena have come to light, encompassing structural phase transitions observed in metallic gold and hydrocarbon materials under shock compression, new insights into electronic quantum dynamics within ultrafast laser fields, and the revelation of H-He immiscibility within planetary interiors. These findings, coupled with the accompanying physical diagram, will provide novel perspectives that propel the scientific exploration of matters under extreme conditions forward.