The development of earth-abundant catalysts for selective electrochemical CO₂ conversion is a central challenge. Cu–Sn bimetallic catalysts can yield selective CO₂ reduction toward either CO or formate. This study presents oxide-derived Cu–Sn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO₂ reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X-ray absorption spectroscopy for in situ study, and quasi in situ X-ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu⁰ under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO-selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate-selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn⁰ and Sn oxide species. Density functional theory simulations suggest that Sn^δ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn⁰ sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts.