The purpose of this substudy is to assess the efficacy and safety of ifinatamab deruxtecan (I-DXd), given alone or with other treatments in participants with metastatic castration-resistant prostate cancer (mCRPC). The goals of this study are to learn about: * The safety of the study treatment and if people tolerate it. * A safe dose level of I-DXd that can be used with other treatments. * Participant levels of prostate specific antigen (PSA) during treatment.