Trial of Mosunetuzumab (BTCT4465A) as Consolidation Therapy in Participants With Diffuse Large B-Cell Lymphoma Following First-Line Immunochemotherapy and as Monotherapy or in Combination With Polatuzumab Vedotin in Elderly/Unfit Participants With Previously Untreated Diffuse Large B-Cell Lymphoma

Inclusion Criteria for All Cohorts

* At least one bi-dimensionally measurable nodal lesion, defined as \> 1.5 cm in its longest dimension, or one bi-dimensionally measurable extranodal lesion, defined as \> 1.0 cm in its longest diameter
* Adequate hematologic function
* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2; with the exception of South Korea, where participants 80 years or older with ECOG \>/= 2 will not be eligible

Inclusion Criteria Specific to Cohort A

Participants in Cohort A must also meet the following criteria for study entry:

* Histologically confirmed DLBCL according to World Health Organization (WHO) 2016 expected to express the cluster of differentiation-20 (CD20) antigen
* One prior therapy with any systemic anthracycline-based chemoimmunotherapy containing regimen for previously untreated DLBCL
* Best response of SD or PR to prior systemic chemoimmunotherapy at the end of induction treatment in accordance with the Lugano 2014 criteria

Inclusion Criteria Specific to Cohorts B and C

Participants in Cohorts B and C must also meet the following criteria for study entry:

* Previously untreated, histologically confirmed, DLBCL according to WHO 2016 classification
* Age \>/= 80 years, or
* Age 65-79 years and considered ineligible for chemoimmuotherapy (R-CHOP) with at least one of the following: Impairment in at least two activity of daily living (ADL) components as defined in the protocol; impairment in at least two instrumental ADL components as defined in the protocol; cumulative illness rating scale - geriactic (CIRS-G) score of at least one cormorbidity with a severity score of 3-4 (not including lymphoma and hematologic deficiencies due to lymphoma) or a score of 2 in \>/= 8 comorbidities; impairment in cardiac function, renal function, liver function, or other comorbidities such that the participant is unfit for full-dose immunochemotherapy, such as rituximab plus cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP)
* Participants with an initial ECOG performance status of 3 may be considered during screening if the performance status is DLBCL-related and if pre-phase treatment during the screening phase (not more than 100 mg/day up to 7 days prior to Cycle 1 Day 1) results in an improvement of ECOG performance status to \</= 2 prior to enrollment

Exclusion Criteria for All Cohorts

Participants who meet any of the following criteria will be excluded from study entry:

* Transformed lymphoma
* CNS lymphoma
* Prior treatment with mosunetuzumab
* Prior stem cell transplant (autologous and allogeneic)
* History of confirmed progressive multifocal leukoencephalopathy (PML)
* Known or suspected chronic active Epstein Barr virus (CAEBV), hepatitis B, hepatitis C (HCV), or Human Immunodeficiency Virus (HIV)
* Known or suspected history of hemophagocytic lymphohistiocytosis (HLH)
* Positive SARS-CoV-2 antigen or PCR test within 30 days prior to Cycle 1 Day 1
* Prior solid organ transplantation
* Current or past history of central nervous system (CNS) disease, such as stroke, epilepsy, CNS vasculitis, or neurodegenerative disease
* Clinically significant history of liver disease
* Prior treatment with radiotherapy within 2 weeks prior to Cycle 1, Day 1 (C1D1)
* Significant cardiovascular disease

Exclusion Criteria Specific to Cohort A

Participants in Cohort A who meet the following criteria will be excluded from study entry:

\- Prior anti-lymphoma treatment with chemotherapy, immunotherapy, or biologic therapy 4 weeks prior to C1D1

Exclusion Criterion Specific to Cohorts B and C

Participants in Cohorts B and C who meet the following criterion will be excluded from study entry:

\- Prior treatment for DLBCL with chemotherapy, immunotherapy, and biologic therapy

Exclusion Criteria Specific to Cohort C

Participants in Cohort C who meet the following criteria will be excluded from study entry:

\- Current Grade \>1 peripheral neuropathy by clinical examination or demyelinating form of Charcot-Marie-Tooth disease