This study is for subjects that have been diagnosed with mantle cell lymphoma that has spread and has not responded to treatment. This study is testing an "investigational" (not yet FDA approved) study drug called glofitamab. The purpose of this study is to compare the effects, good or bad, of glofitamab (experimental arm) versus bendamustine plus rituximab (BR) or rituximab plus lenalidomide (R-Len;the control arm) on subjects with relapsed/refractory mantle cell lymphoma. Your total time in the study and the number of assessments in the follow up visits, will depend on how your MCL responds to study treatment. This could range from 1 day to more than 24 months. The screening period may last up to 28 days (4 weeks) and may involve more than one visit to the clinic.