This study is for adults with chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) whose disease has returned after treatment or has not responded to treatment, and who have previously received a Bruton tyrosine kinase (BTK) inhibitor.

This study is testing an investigational drug called BGB-16673. "Investigational" means it has not been approved by the United States Food and Drug Administration (FDA).

This is a randomized study, which means participants are assigned by chance (like flipping a coin) to receive either BGB-16673 or pirtobrutinib.The primary purpose of this study is to compare the safety and effectiveness of BGB-16673 with pirtobrutinib in patients with relapsed or refractory CLL or SLL.The study drugs are taken by mouth. Participants will attend regular study visits for health assessments, laboratory tests, and disease evaluations and will remain in the study until their disease worsens, they experience unacceptable side effects, choose to leave the study, or the study ends.