A Study to Evaluate Glofitamab Monotherapy and Glofitamab + Chemoimmunotherapy in Pediatric and Young Adult Participants With Relapsed/Refractory Mature B-Cell Non-Hodgkin Lymphoma

Inclusion Criteria: * Age 6 months to \< 18 years at the time of signing Informed Consent for Cohort A Part 1 and Cohort B of the study, and age 6 months to \< 30 years old at the time of signing Informed Consent for Cohort A Part 2 of the study * Histologically re-confirmed diagnosis, via tissue biopsy, or bone marrow aspirate, pleural effusion, or ascites, prior to study entry of aggressive mature B-NHL that expresses CD20 (reconfirmed by IHC or flow cytometry if IHC is not possible), including BL, BAL (mature B-cell leukemia FAB L3), DLBCL, and PMBCL, at the time of first R/R disease for Cohort A and second or greater R/R disease for Cohort B * Refractory or relapsed disease (i.e., prior treatment was ineffective or intolerable) following first-line standard-of-care chemoimmunotherapy for Cohort A and following at least two prior systemic chemoimmunotherapy regimens and who have exhausted all available established therapies for Cohort B * Measurable disease, defined as: At least one bi-dimensionally measurable nodal lesion, defined as \> 1.5 cm in its longest dimension, or at least one bi dimensionally measurable extranodal lesion, defined as \> 1.0 cm in its longest dimension; or percentage of bone marrow involvement with lymphoma cells defined by cytomorphological analysis of bone marrow aspirates * Adequate performance status, as assessed according to the Lansky or Karnofsky Performance Status scales: Participants \< 16 years old: Lansky Performance Status ≥ 50%; Parti…