This trial is testing a new cancer drug called Trastuzumab deruxtecan in patients with advanced HER2-positive breast cancer that has not responded to other treatments. The drug targets cancer cells with a specific marker and delivers a powerful drug to kill them. The study includes patients with and without brain metastasis to see how well the drug works and how safe it is. Trastuzumab deruxtecan is a new type of treatment that has shown impressive results in patients who have already received many other treatments.