This is a study for patients with relapsed/refractory (R/R) chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL) who have previously received treatment with a BTK inhibitor (covalent and non-covalent) and a BCL-2 inhibitor. The main purpose of this study is to test if NX-5948 (bexobrutideg) works to treat patients with CLL/SLL. Participation could last up to 5 years, and possibly longer, if the disease does not progress.