Substudy 01A is part of a larger research study that is testing experimental treatments for metastatic castration-resistant prostate cancer (mCRPC). The larger study is the umbrella study (U01). The goal of substudy 01A is to evaluate the safety and efficacy of opevesostat-based treatment combinations, or as a single agent, in participants with mCRPC. This substudy will have two phases: a safety lead-in phase and an efficacy phase. The safety lead-in phase will be used to evaluate the safety and tolerability, and to establish a recommended Phase 2 dose (RP2D) for the opevesostat-based treatment combinations. There will be no hypothesis testing in this study.