Study of Tarlatamab in Combination With YL201 With or Without Anti-programmed Death Ligand 1 (PD-L1) in Participants With Extensive Stage (ES) Small Cell Lung Cancer (SCLC)

Inclusion Criteria: * Participants ≥ 18 years of age (or legal adult age within country) at time of signing informed consent. * Participants with histologically or cytologically confirmed ES-SCLC. * For Parts 1 and 2, participant must have ES-SCLC that has progressed or recurred following at least 1 line of platinum-based anti-cancer therapy. * For Part 3, participants must have ES-SCLC and no prior systemic treatment for ES SCLC other than 1 cycle of platinum-based chemotherapy, etoposide, and PD-(L)1 inhibitor in the first-line setting. * At least 1 measurable lesion as defined by RECIST 1.1. * Participants must have adequate organ function (cardiac, pulmonary, kidney, bone marrow, and liver). Exclusion Criteria: * Prior delta-like ligand 3 (DLL3) or B7 homolog 3 (B7-H3) targeted therapy. * Prior exposure to topoisomerase I inhibitors or antibody-drug conjugate (ADC) with topoisomerase I inhibitor payload. * Symptomatic central nervous system (CNS) metastases. Note: Participants with asymptomatic brain metastases are eligible as defined in the protocol. * History of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required corticosteroids, current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out by imaging at screening. * Baseline requirement of supplemental oxygen.