Extensive stage small cell lung cancer (ES-SCLC) is an aggressive type of cancer that can rapidly spread to other areas of the body. This type of cancer currently has limited treatment options. The purpose of this study is to determine the efficacy and safety of lurbinectedin (a type of chemotherapy) when combined with atezolizumab (an immunotherapy treatment) as a maintenance treatment for those with ES-SCLC. This combination treatment will be compared a treatment of atezolizumab alone. Participants will be randomly chosen to receive a combination of lurbinectedin and atezolizumab or atezolizumab alone. They will not know which treatment they are receiving. The treatments are delivered by IV infusion. Participants also may be asked to provide blood samples and have a physical exam.