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

Overall Survival (OS)

OS is defined as time from the date of randomization to the date of death due to any cause.

Time frame: Approximately 36 Months

Progression-Free Survival (PFS) in Participants with Prior Exposure to Noncovalent Bruton Tyrosine Kinase Inhibitor(s) (ncBTKi) by IRC

PFS is defined as time from the date of randomization to the date of first disease progression or death, whichever occurs first, as determined by IRC using modified 2018 iwCLL for participants with prior exposure to ncBTKi.

Time frame: Approximately 36 Months

PFS by the Investigator Assessment

PFS is defined as time from the date of randomization to the date of first disease progression or death, whichever occurs first, as determined by the investigator using modified 2018 iwCLL criteria for participants with CLL and Lugano classification for participants with SLL

Time frame: Approximately 36 Months

Overall Response Rate (ORR) by IRC and Investigator Assessment

ORR is defined as the percentage of participants with best overall response of complete response (CR), complete response with incomplete bone marrow recovery (Cri), nodular partial remission (nPR), or partial response (PR) as assessed by the IRC or by the investigator.

Time frame: Approximately 36 Months

Rate of Partial Response with Lymphocytosis (PR-L) or Higher Determined by IRC and by Investigator Assessment

Rate of PR-L or higher is defined as the percentage of participants with a best overall response of CR, CRi, nPR, PR, or PR-L as assessed by the IRC or by the Investigator

Time frame: Approximately 36 Months

Duration of Response (DOR) by IRC and Investigator Assessment

DOR is defined as the time from initial response to disease progression or death, whichever occurs first, as assessed by the IRC or by the investigator.

Time frame: Approximately 36 Months

Time to Next Anti-CLL/SLL Treatment (TTNT)

TTNT is defined as the time from the date of randomization to the date of next anti-CLL/SLL treatment.

Time frame: Approximately 36 Months

Number of Participants With Treatment-emergent Adverse Events (TEAEs) and Treatment Emergent Serious Adverse Events (TESAEs)

Safety will be assessed by monitoring and recording of all treatment emergent adverse events (AEs) graded by National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) v5.0.

Time frame: Approximately 36 Months

Change from Baseline in European Organization for Research and Treatment of Cancer Quality of Life Cancer Questionnaire -Core 30 (EORTC QLQ-C30) Global Health Status/QoL (GHS) and Physical Functioning Scales

The EORTC QLQ-30 contains 30 questions that incorporate 5 functional scales (physical functioning, role functioning, emotional functioning, cognitive functioning, and social functioning), 1 global health status scale, 3 symptom scales(fatigue, nausea and vomiting, and pain), and 6 single items (dyspnea, insomnia, appetite loss, constipation, diarrhea, and financial difficulties). The participant answers questions about their health during the past week. There are 28 questions answered on a 4-point scale where 1 =Not at all (best) to 4 =Very Much (worst) and 2 questions answered on a 7-point scale where 1 =Very poor (worst) to 7 =Excellent (best). Higher scores in GHS and functional scales indicate better quality of life.

Time frame: Approximately 24 Months

Change from baseline in EORTC Quality of Life Questionnaire - Chronic Lymphocytic Leukemia Module 17 Items (QLQ-CLL17) Symptom Burden and Physical Condition Scales

The symptom burden and physical condition will be measured by QLQ-CLL17. EORTC QLQ-CLL17 comprises 17 items grouped into 3 multi-item scales: 1) symptom burden, 2) physical condition/fatigue, and 3) worries/fears about health and functioning. Each question is rated using a 4-point response scale ("not at all," "a little," "quite a bit," and "very much") and the recall period for all items is the past 7 days. The scores of each subscale are calculated based on the EORTC's protocol and then transformed to a 0 to 100 scale. Higher scores represent higher levels of symptom burden, physical condition/fatigue, or worries/fears about health and functioning.

Time frame: Approximately 24 Months