This study will evaluate the combination of a fixed dose pembrolizumab/vibostolimab co-formulation (MK-7684A) with etoposide/platinum chemotherapy followed by MK-7684A compared to the combination of atezolizumab with etoposide/platinum chemotherapy followed by atezolizumab in the first-line treatment of Extensive-Stage Small Cell Lung Cancer (ES-SCLC). The primary hypothesis is, with respect to overall survival, MK-7684A in combination with the background therapy of etoposide/platinum followed by MK-7684A, is superior to atezolizumab in combination with the background therapy of etoposide/platinum followed by atezolizumab.