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

PFS as Assessed by the Investigator

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 the Lugano Classification for patients with R/R SLL.

Time frame: Approximately 12 Months

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 21 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 23 Months

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

Rate of PR-L or higher is defined as the percentage of participants with a best overall response of complete response (CR), complete response with incomplete bone marrow recovery (Cri), nodular partial remission (nPR), partial response (PR), or partial response with lymphocytosis (PR-L) as assessed by the Investigator.

Time frame: Approximately 23 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 9 Months

Time to Response (TTR) by IRC and Investigator Assessment

TTR is defined as the time from randomization until initial response, as assessed by the IRC or by the investigator.

Time frame: Approximately 6 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 14 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) and laboratory abnormalities graded by National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE) v5.0.

Time frame: Approximately 14 Months

Change from baseline in European Organization of Research and Treatment of Cancer (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: Baseline and up to approximately 23 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 (GHS)/Quality of Life (QoL) 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). The scores of each subscale are calculated based on the EORTC's protocol and then transformed to a 0 to 100 scale. Higher scores in GHS and functional scales indicate better quality of life.

Time frame: Baseline and up to approximately 23 Months