An Open-Label Phase lB/II Study of Glofitamab and Atezolizumab or Polatuzumab Vedotin in Adult Patients With Relapsed/Refractory B-Cell Non-Hodgkin's Lymphoma

Main Inclusion Criteria * Histologically-confirmed hematologic malignancy that is expected to express CD20 (Relapsed after or refractory to respond to at least one prior treatment regimen; no available treatment options that are expected to prolong survival or patients refusing chemotherapy or autologous stem cell transplant (SCT)) * Dose-escalation: Grades 1-3b relapsed or refractory (R/R) follicular lymphoma (FL) or marginal zone lymphoma (MZL) (nodal; extra-nodal; or splenic), diffuse large B-cell lymphoma (DLBCL), primary mediastinal large B-cell lymphoma (PMBCL), high-grade B-cell lymphoma (HGBCL) with MYC and BCL2 and/or BCL6 rearrangements (double-hit lymphoma), HGBCL not otherwise specified (NOS), DLBCL arising from FL (transformed FL) * Dose-expansion: R/R LBCL, including DLBCL NOS, DLBCL arising from FL (transformed FL), PMBCL, HGBCL with MYC and BCL2 and/or BCL6 rearrangements (i.e., double-hit and triple-hit lymphomas), and HGBCL NOS * At least one measurable target lesion * Fresh pre-treatment biopsy, but if this cannot be taken, a previous archived biopsy from metastatic lesion can be taken as replacement if it is not older than 6 months and not confounded by major events (progression, treatment) * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2 * Adequate organ function (liver, hematological, renal) * Negative test results for hepatitis B virus (HBV), hepatitis C virus (HCV), and human immunodeficiency virus (HIV) Inclusion Criteria Specific to Imaging Substudy * At least two measurable target lesions * Able to provide two fresh tumor biopsies (baseline and on-treatment) Main Exclusion Criteria * Participants with Chronic Lymphocytic Leukemia (CLL), acute lymphoblastic leukemia (ALL), lymphoblastic lymphoma, Richter's transformation, CD20-positive ALL, Burkitt lymphoma, or lymphoplasmacytic lymphoma * Current \> Grade 1 peripheral neuropathy (only for participants being treated in the polatuzumab vedotin arm) * Patients with known active infection, or reactivation of a latent infection within 4 weeks prior to Obinutuzumab (Gpt) infusion * Patient with history of confirmed progressive multifocal leukoencephalopathy (PML) * History of leptomeningeal disease * Current or past history of central nervous system (CNS) lymphoma * Current or past history of CNS disease * Major surgery or significant traumatic injury \</=28 days prior to Gpt infusion * Significant cardiovascular disease or significant pulmonary disease * Active or history of autoimmune disease or immune deficiency (with exceptions, e.g. hypothyroidism and Diabetes mellitus Type 1) * History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g. bronchiolitis obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest computed tomography (CT) scan * Treatment with any other standard anti-cancer radiotherapy / chemotherapy including investigational therapy within 4 weeks prior to Gpt infusion * Prior solid organ transplantation * Prior allogenic stem cell transplant (SCT) * Autologous SCT within 100 days prior to Gpt infusion * Documented refractoriness to an obinutuzumab-monotherapy regimen * Prior treatment with anti-cancer/lymphoma therapies and systemic immunotherapeutic/immunostimulating agents within 4 weeks or 5 half-lives of the drug, whichever is shorter, prior to Gpt infusion * Any history of immune related \>/= Grade 3 adverse events (AE) with the exception of endocrinopathy managed with replacement therapy * Ongoing corticosteroid use \>25 milligrams/day of prednisone or equivalent within 4 weeks prior to and during study treatment * Treatment with systemic immunosuppressive medication * Administration of a live, attenuated vaccine within 4 weeks prior to Gpt infusion or anticipation that such a live attenuated vaccine will be required during the study or within 5 months after last dose of study treatment Exclusion Criteria Specific to Imaging Substudy * Circulating lymphoma cells, defined by out of range (high) absolute lymphocyte count and/or the presence of abnormal/malignant cells in the peripheral blood differential signifying circulating lymphoma cell * Participants who have had splenectomy or functional asplenia that could compromise protocol objectives