A Study of BGB-16673 Compared to Investigator's Choice in Participants With Relapsed/Refractory Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma Previously Exposed to Covalent Bruton Tyrosine Kinase (BTK) Inhibitors

The purpose of this study is to investigate the efficacy and safety of BGB-16673 compared with investigator's choice (bendamustine plus rituximab or high-dose methylprednisolone plus rituximab) in participants with relapsed/refractory (R/R) chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) previously exposed to covalent Bruton tyrosine kinase inhibitor(s) (cBTKi).

Inclusion Criteria: 1. Confirmed diagnosis of CLL/SLL, requiring treatment, based on 2018 international workshop on chronic lymphocytic leukemia (iwCLL) criteria. 2. Previously received treatment for CLL/SLL with a covalent BTKi. 3. Measurable disease by computer tomography/magnetic resonance imaging for patients with SLL. 4. Eastern Cooperative Oncology Group (ECOG) performance status 0-2 5. Adequate bone marrow function 6. Adequate kidney and liver function 7. Adequate blood clotting function Exclusion Criteria: 1. Known prolymphocytic leukemia or history of, or currently suspected, Richter's transformation 2. 2\. Prior autologous stem cell transplant (unless ≥ 3 months after transplant) or chimeric antigen receptor-T cell (unless ≥ 6 months after cell infusion) 3. History of severe allergic reactions or hypersensitivity to the active ingredient and excipients of study treatment (BGB-16673, bendamustine, or rituximab) 4. Current or history of central nervous system involvement 5. History of ischemic stroke or intracranial hemorrhage within 6 months before first dose of study drug 6. History of confirmed progressive multifocal leukoencephalopathy. 7. Active fungal, bacterial, and/or viral infection requiring parenteral systemic therapy 8. Clinically significant cardiovascular disease NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.