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 internationalworkshop 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 patientswith 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'stransformation

2. 2. Prior autologous stem cell transplant (unless ≥ 3 months after transplant) orchimeric antigen receptor-T cell (unless ≥ 6 months after cell infusion)

3. History of severe allergic reactions or hypersensitivity to the active ingredientand 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 firstdose of study drug

6. History of confirmed progressive multifocal leukoencephalopathy.

7. Active fungal, bacterial, and/or viral infection requiring parenteral systemictherapy

8. Clinically significant cardiovascular disease

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.