An Open-Label, Multicenter Study Evaluating the Safety, Efficacy, and Pharmacokinetics of Mosunetuzumab in Combination With Tiragolumab With or Without Atezolizumab in Participants With B-Cell Non-Hodgkin Lymphoma

Inclusion Criteria: * Aged \>/= 18 years * Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 * Life expectancy of at least 12 weeks * Histologically documented FL or DLBCL that has relapsed or failed to respond to at least two prior systemic treatment regimens and for which no suitable therapy of curative intent or higher priority exists (e.g., standard chemotherapy, ASCT, CAR T cells) * At least one bi-dimensionally measurable (\> 1.5 cm) nodal lesion, or at least one bi-dimensionally measurable (\> 1.0 cm) extranodal lesion * Participants with FL (including trFL) for whom a bone marrow biopsy and aspirate can be collected * Adequate hematologic and organ function Exclusion Criteria: * Received any of the following treatments prior to study entry: mosunetuzumab or other CD20/CD3-directed bispecific antibodies; tiragolumab or other anti-TIGIT agent; allogenic SCT; solid organ transplantation * Currently eligible for autologous SCT * Current or past history of CNS lymphoma or leptomeningeal infiltration * History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibody therapy (or recombinant antibody-related fusion proteins) * Contraindication to atezolizumab (if applicable) or tocilizumab * Clinically significant toxicities from prior treatment have not resolved to Grade \</= 1 (per NCI CTCAE v5.0) prior to the first study drug administration with exceptions defined by the protocol * Treatment-emergent immune-mediated adverse events associated with prior immunotherapeutic agents as defined by the protocol * Evidence of any significant, concomitant disease as defined by the protocol * Major surgery within 4 weeks prior to first study treatment administration, with the exception of protocol-mandated procedures (e.g., tumor biopsies and bone marrow biopsies) * Significant cardiac, pulmonary, CNS, or liver disease, or known active infections * History of other malignancy that could affect compliance with the protocol or interpretation of results * History of autoimmune disease with exceptions as defined in the protocol