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

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, rituximab, chlorambucil, or obinutuzumab) 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.