This trial is testing two different treatment combinations for patients with extensive-stage small cell lung cancer who have not received any prior treatment. One group will receive a new drug called Serplulimab along with standard chemotherapy, while the other group will receive an existing drug called Atezolizumab with the same chemotherapy. Both immunotherapy drugs aim to help the immune system fight cancer, and the chemotherapy drugs work by killing cancer cells. Atezolizumab has been shown to improve survival in small-cell lung cancer when combined with chemotherapy, changing the standard first-line therapy.