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

The purpose of this study is to investigate the efficacy and safety of BGB-16673 compared with investigator's choice (idelalisib plus rituximab \[for CLL only\] or bendamustine plus rituximab or venetoclax plus rituximab retreatment) in participants with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) previously exposed to both BTK inhibitors (BTKi) and BCL2 inhibitors (BCL2i).

Inclusion Criteria: 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 Exclusion Criteria: 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.