Each Part will be initiated with a dose-escalation phase. During the open-label, escalation phases, increasing doses of venetoclax with fixed doses of daratumumab and dexamethasone (Part 1a) or daratumumab, bortezomib, and dexamethasone (Part 2a) will be administered. The dose-escalation phases will be followed by either a randomized, and blinded (Part 1b) expansion phase or single-arm and open-label (Part 2b) expansion phase.