A Clinical Study to Find the Optimal Dose of an Investigational Treatment Called BNT323 When Used in Combination With Another Investigational Treatment, BNT327, and to Test if That Combination Treatment is Safe and Beneficial for Patients With Advanced Breast Cancer

Key Inclusion Criteria (applicable to all participants and all parts unless otherwise specified): * Have pathologically documented BC that: * Is locally advanced, unresectable or metastatic. * Has a confirmed HER2 status as determined by the local laboratory as standard of care testing prior to study screening (Part 1, Part 2 Cohorts 2 and 4) or the central laboratory (Part 2, Cohorts 1 and 3) from the most recently collected pre-randomization tumor sample. * Has a documented history of HER2 expression consistent with the subgroup definitions (i.e., HER2-low, HER2-ultralow, HER2-null, HER2-positive, or TNBC) as per current American Society of Clinical Oncology/College of American Pathologists guidelines. * Have measurable disease defined by RECIST v1.1. * Has left ventricular ejection fraction ≥55% by either echocardiography or multi-gated acquisition (scanning) within 28 days before randomization/enrollment. Key Exclusion Criteria: * Have history of small bowel obstruction requiring hospitalization within the past 3 months prior to the first dose of IMP. * Have an uncontrolled intercurrent illness that would limit compliance with study requirement or substantially increase risk of incurring adverse events. * Have clinically uncontrolled pleural effusion, ascites or pericardial effusion requiring drainage, peritoneal shunt, or cell-free concentrated ascites reinfusion therapy within 2 weeks prior to randomization/enrollment. * Have a history of (non-infectious) inte…