Study of Selinexor in Combination With Backbone Treatments or Novel Therapies In Participants With Relapsed or Refractory (RR) Diffuse Large B-Cell Lymphoma (DLBCL)

Inclusion Criteria: 1. Participants greater than or equal to (≥) 18 years of age. 2. Have pathologically confirmed relapsed/refractory (RR) DLBCL, not otherwise specified (NOS). 3. Participants with High Grade B-cell Lymphoma (HGBL) are allowed in Phase 2 only. 4. Prior lines of systemic therapy for the treatment of DLBCL: * For Arms A, B, C, E, F, G, H: Participants must have received at least 1 but no more than 3 prior lines of systemic therapy for the treatment of DLBCL. * For Arm D (S-R-GemOx) participants must have received at least 1 but not more than 2 lines of systemic therapy. 5. Positron emission tomography (PET) positive measurable disease per the Lugano Classification 2014, having at least 1 node with longest diameter (LDi) greater than (\>) 1.5 centimetres (cm) or 1 extranodal lesion with LDi \>1 cm. 6. Adequate bone marrow function at Screening. 7. Circulating lymphocytes less than or equal to (≤) 50 \* 109/L. 8. Adequate liver and kidney function. 9. Eastern Cooperative Oncology Group (ECOG) performance status of ≤2. 10. An estimated life expectancy of \>6 months at Screening. 11. Participants with primary refractory disease defined as no response or relapse within 6 months after ending first-line treatment will be allowed on study (up to 20 percentage \[%\] of enrolled participants in each Phase). 12. Male participants, and female participants of childbearing potential must agree to use highly effective methods of contraception during the duration of the study and will continue following the last dose of study treatment for the longest duration stated on the label of each of the given drugs (depending on each arm). 13. Female participants of childbearing potential must have a negative serum pregnancy test at Screening. Female participants of childbearing potential in the S-LR arm and the S-LT arm must have 2 negative pregnancy tests before Lenalidomide treatment (Non-Childbearing potential: Age \>50 years and naturally amenorrhoeic for \>1 year, or previous bilateral salpingo-oophorectomy, or hysterectomy). 14. Participants with active hepatitis B Virus (HBV) are eligible if antiviral therapy for hepatitis B has been given for \>8 weeks and viral load is \<100 international units per milliliter (IU/mL); participants with untreated hepatitis C Virus (HCV) are eligible if viral load is negative per institutional standard; participants with human immunodeficiency virus (HIV) are eligible if cluster of differentiation 4 (CD4+) T-cell counts ≥350 cells per microliter (cells/μL), viral load is negative and no history of acquired immunodeficiency syndrome (AIDS)-defining opportunistic infections in the last year. Exclusion Criteria: 1. DLBCL with mucosa-associated lymphoid tissue (MALT) lymphoma; composite lymphoma (Hodgkin lymphoma + NHL); Gray zone lymphoma; DLBCL transformed from Chronic Lymphocytic Leukemia (Richter Syndrome); Primary mediastinal large B-cell lymphoma (PMBCL); T-cell rich large B-cell lymphoma. 2. Previous treatment with selinexor or other XPO1 inhibitors. 3. Contraindication to any drug contained in the different treatment arms. 4. Use of any standard or experimental anti-DLBCL therapy (including nonpalliative radiation, chemotherapy, immunotherapy, radio-immunotherapy, or any other anticancer therapy) \<21 days prior to Cycle 1 Day 1 (C1D1). Low dose steroids \<30 mg prednisone (or equivalent) and palliative radiotherapy are permitted. 5. Received strong cytochrome P450 3A (CYP3A) inhibitors ≤7 days prior to Day 1 dosing or strong CYP3A inducers ≤14 days prior to Day 1 dosing. 6. Any AE, by Cycle 1 Day 1 (C1D1), which has not recovered to Grade ≤1 (CTCAE, v. 5.0), or returned to baseline, related to the previous DLBCL therapy, except alopecia. 7. Major surgery \<14 days of C1D1. 8. Autologous stem cell transplant (SCT) \<100 days or allogeneic SCT \<180 days prior to C1D1 or active graft-versus-host disease after allogeneic SCT (or cannot discontinue graft versus host disease \[GVHD\] treatment or prophylaxis). 9. Prior chimeric antigen receptor T cell (CAR-T cell) infusion at any time (Phase 1 only); prior CAR-T cell infusion ≤120 days prior to C1D1 (Phase 2 only). 10. Any life-threatening illness, medical condition, or organ system dysfunction which, in the Investigator's opinion, could compromise the participant's safety, or able to comply with the study procedures. 11. Uncontrolled (i.e., clinically unstable) infection requiring parenteral antibiotics, antivirals, or antifungals within 7 days prior to first dose of study treatment; however, prophylactic use of these agents is acceptable (including parenteral). 12. Inability to swallow tablets, malabsorption syndrome, or any other GI disease or dysfunction that could interfere with absorption of study treatment. 13. Breastfeeding women or pregnant women. 14. Inability or unwillingness to sign informed consent form. 15. In the opinion of the Investigator, participants who are below their ideal body weight and would be unduly impacted by changes in their weight. 16. Known allergy to any of the drug planned to be given. The following are Arm Specific exclusion criteria: 17. Arm B (S-PR): Serum total bilirubin \>1.5 \* ULN, Neuropathy Grade ≥2 (CTCAE, v5.0). 18. Arm C (S-PBR): Serum total bilirubin \>1.5 \* ULN, Neuropathy Grade ≥2 (CTCAE, v5.0). 19. Arm D (S-R-GemOx): Neuropathy Grade 2≥ (CTCAE, v5.0) interstitial lung disease or pulmonary fibrosis.