A Study of Atezolizumab Plus Tiragolumab and Atezolizumab Plus Placebo as First-Line Treatment in Participants With Recurrent/Metastatic PD-L1 Positive Squamous Cell Carcinoma of the Head and Neck

Key Inclusion Criteria: * Histologically or cytologically confirmed recurrent/metastatic SCCHN involving the oropharynx, oral cavity, larynx, or hypopharynx, that is considered incurable by local therapies * Known results from human papillomavirus (HPV) status test for oropharyngeal carcinoma * No prior systemic therapy for metastatic and/or recurrent SCCHN * Measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 * Tumor PD-L1 expression as determined by PD-L1 immunohistochemistry assay * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Life expectancy \>=12 weeks Key Exclusion Criteria: * Disease suitable for local therapy with curative intent * Progressive or recurrent disease within 6 months of the last dose of curative intent systemic treatment for locally advanced SCCHN * Rapidly progressing disease in the opinion of the treating investigator * Grade \>=2 unresolved toxicity related to surgery or other prior therapies * Symptomatic, untreated, or actively progressing central nervous system (CNS) metastases * History of leptomeningeal disease * Active or history of autoimmune disease or immune deficiency * History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan * History of additional malignancy other than SCCHN within 5 years prior to randomization * Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including anti-CTLA-4, anti-TIGIT, anti-PD-L1, and anti-PD-1 therapeutic antibodies * Treatment with systemic immunostimulatory agents or systemic immunosuppressive medication * Pregnancy or breastfeeding