Researchers are looking for new ways to treat metastatic castration-resistant prostate cancer (mCRPC). Researchers have designed a study medicine called ifinatamab deruxtecan (also called I-DXd or MK-2400) to treat mCRPC. The goal of this study is to learn if people who receive I-DXd live longer overall and live longer without the cancer growing or spreading than people who receive chemotherapy.