A Study of BGB-16673 Compared to Investigator's Choice in Participants With Chronic Lymphocytic Leukemia or Small Lymphocytic Lymphoma Previously Exposed to Both Bruton Tyrosine Kinase (BTK) and B-cell Leukemia/Lymphoma 2 Protein (BCL2) Inhibitors

Who May Qualify: 1. Confirmed diagnosis of CLL or SLL, requiring treatment, based on 2018 international workshop on chronic lymphocytic leukemia (iwCLL) criteria. 2. Previously received treatment for CLL/SLL with both a BTKi and a BCL2i. 3. Participants with SLL must have measurable disease by computer tomography (CT)/magnetic resonance imaging (MRI) 4. Eastern Cooperative Oncology Group (ECOG) score 0, 1, or 2 5. Adequate liver function 6. Adequate blood clotting function Who Should NOT Join This Trial: 1. Known prolymphocytic leukemia or history of, or currently suspected, Richter's transformation 2. Prior autologous stem cell transplant or chimeric antigen receptor-T cell therapy in the last 3 months 3. Known central nervous system involvement 4. Prior exposure to any BTK protein degraders 5. Active fungal, bacterial and/or viral infection requiring parenteral systemic therapy 6. Clinically significant cardiovascular disease NOTE: Other protocol defined Inclusion/Exclusion criteria may apply. Always talk to your doctor about whether this trial is right for you.