A Phase 3b, Multicenter, Global, Interventional, Open-label Study of Trastuzumab Deruxtecan (T-DXd), an Anti-HER2-Antibody Drug Conjugate (ADC), in Subjects Who Have Unresectable and/or Metastatic HER2-low or HER2 Immunohistochemistry (IHC) 0 Breast Cancer (DESTINY-Breast15)

Inclusion Criteria: * Sign and date the main informed consent form * Must agree to provide a newly obtained or archival baseline biopsy from primary and/or metastatic lesion. * Pathologically documented Breast Cancer (BC) tumor * Is unresectable and/or metastatic. * Is hormone receptor-negative or hormone receptor-positive. * Must include percentage of positively stained cells to characterize if hormone receptor-positive or -negative. * Has confirmed HER2 IHC 1+ or IHC 2+/ISH- (HER2-low) status or HER2 IHC 0 status as determined according to ASCO CAP 2018 guidelines1 based on sample collected during Tissue Screening as described above. * Was never previously HER2-positive (IHC 3+ or IHC 2+/ISH+) on prior pathology testing (per ASCO CAP guidelines). * Was never previously treated with anti-HER2 therapy in the metastatic setting. * Has had at least one and up to two prior lines of therapy in the metastatic setting. * In participants with hormone receptor-positive HER2-low metastatic BC (Cohort 3): * Has recurrent disease \<2 years from the initiation of adjuvant ET OR * Has disease progression on CDK4/6 inhibitor-based regimen within 12 months of completion of adjuvant therapy with a CDK4/6 inhibitor OR * Has disease progression within the first 12 months of CDK4/6 in the first line metastatic setting * Presence of at least one measurable lesion based on computed tomography or magnetic resonance imaging. * Participants with brain metastases are allowed in the study. The brain lesion(s) should be small (\<2 cm), untreated, asymptomatic, not requiring urgent medical intervention, and are asymptomatic and clinically stable. * Has an Eastern Cooperative Oncology Group performance status of 0 or 1. * Has a minimum life expectancy of 12 weeks at Screening. * Has a left ventricular ejection fraction ≥50% within 28 days before enrollment. * Has adequate organ and bone marrow function within 28 days before enrollment. * Has adequate treatment washout period before enrollment. * Male and female subjects of reproductive/childbearing potential must agree to use a highly effective form of contraception. Exclusion Criteria: * Prior treatment with an antibody drug conjugate (ADC). * Uncontrolled or significant cardiovascular disease. * Has a corrected QT interval prolongation. * Has a history of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required steroids, has current ILD/pneumonitis, or where suspected ILD/pneumonitis cannot be ruled out by imaging at Screening. * Has spinal cord compression or clinically active central nervous system metastases. * Has multiple primary malignancies within 3 years, except adequately resected non-melanoma skin cancer, curatively treated in situ disease, other solid tumors curatively treated, or contralateral BC. * Has a history of severe hypersensitivity reactions to either the drug substances or inactive ingredients in the drug product. * Has a history of severe hypersensitivity reactions to other monoclonal antibodies. * Has an uncontrolled infection requiring intravenous (IV) antibiotics, antivirals, or antifungals. * Active primary immunodeficiency, known uncontrolled active human immunodeficiency virus (HIV) infection, or active hepatitis B or C infection. * Has history of receiving a live, attenuated vaccine (messenger RNA and replication-deficient adenoviral vaccines are not considered attenuated live vaccines) within 30 days prior to the first exposure to study drug. * Has unresolved toxicities from previous anticancer therapy, defined as toxicities (other than alopecia) not yet resolved to Grade ≤1 or baseline. * Is pregnant or breastfeeding or planning to become pregnant. * Lung-specific intercurrent clinically significant illnesses. * Any autoimmune, connective tissue, or inflammatory disorders. * Prior complete pneumonectomy.