Symbiotic-Lung-04: A Study to Learn About the Study Medicine Called PF-08634404 in Combination With Chemotherapy in Adult Participants With Extensive-Stage Small Cell Lung Cancer

This study is being done to learn more about a new medicine called PF-08634404 and how well it works when given with chemotherapy to adults with extensive-stage small cell lung cancer (ES-SCLC), a fast-growing type of lung cancer that has spread widely in the body. To join the study, participants must meet the following conditions: * Be 18 years or older. * Have extensive-stage small cell lung cancer confirmed by lab tests. * Have not received chemotherapy or radiation for this type of lung cancer. * Be in good physical condition and have healthy organs based on medical tests. The study has two parts: * In the first part, researchers will check how safe the study medicine is and how well people tolerate it when given with chemotherapy. * In the second part, they will compare study medicine plus chemotherapy to another approved treatment (atezolizumab plus chemotherapy) to see which works better. Participants will receive the treatment through IV infusions (medicine given directly into a vein). The treatment will be given in repeated time periods called cycles. Some participants will continue receiving the study medicine alone after the initial treatment.