This is a study of venetoclax, daratumumab, and dexamethasone with and without bortezomib combination therapy to evaluate safety, tolerability, and efficacy of these combinations in participants with relapsed or refractory multiple myeloma. The study will consist of 2 distinct parts: Part 1 includes participants with t(11;14) positive relapsed/refractory (R/R) multiple myeloma who will receive venetoclax in combination with daratumumab and dexamethasone (VenDd); Part 2 includes participants with R/R multiple myeloma who will receive venetoclax in combination with daratumumab, bortezomib, and dexamethasone (VenDVd).