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.