Previous studies have shown that introducing GBs structures and oxidation states into CuAg systems enhances CH4 production, underscoring the critical role of GBO. However, in the C2+ product pathway, the promoting or inhibiting effects of individual design strategies within complex structures involving GBs, oxidation states, and alloying remain unclear. The decisive factor in the C2+ product pathway remains uncertain. Therefore, in this work, in situ multimodal characterizations reveal that GBs and GBO effects enhance the resistance of Cuδ+ species to electrochemical reduction. In complex GBs assemblies, the oxidation state is decisive in the C2+ pathway, whereas nanoalloy systems tend to favor methane formation. Theoretical calculations indicate that oxidized copper species lower the energy barrier of the rate-determining step, facilitating C–C coupling toward C2+ products. This study pinpoints the origins of catalytic activity, clarify the relationship between design strategies and catalytic performance, and elucidate the dynamic evolution of active sites during ECR.