The main goal of this study is to test a new medicine, Risvutatug Rezetecan also called Ris-Rez. We want to see if this medicine can help people with certain types of cancer, whether its safe to use, how well people tolerate it, and how their bodies handle the drug (how its absorbed and broken down). This research is for adolescents and adults who have either: Osteosarcoma, which is a type of bone cancer, or Soft Tissue Sarcoma, which is a type of cancer that starts in soft body tissues (like muscle, fat, or nerves). In both cancer types the cancer must have already been treated, but has come back or spread, and cant be removed by surgery