A Study of Tacabrutideg (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

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) performance status 0, 1, or 2.
5. Adequate liver function.
6. Adequate blood clotting function.

Exclusion Criteria:

1. Known prolymphocytic leukemia (defined per iwCLL 2018 as prolymphocytes comprising \>55% of peripheral blood lymphocytes), or currently suspected or history of Richter's transformation within the last 5 years.
2. Prior autologous stem cell transplant or chimeric antigen receptor-T cell therapy in the last 6 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.
7. Prior invasive fungal infection, except if patient agrees to receive secondary antifungal prophylaxis during the entire treatment period.
8. Known contraindications to azole antifungal agents, including hypersensitivity reactions.
9. Receiving systemic immunosuppressive therapy for a nonmalignant condition within 28 days before the first dose of study treatment.

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