A Phase 1b/2, Open-label Study to Investigate the Safety and Efficacy of Invikafusp Alfa (STAR0602), a Selective T Cell Receptor (TCR)-Targeting, Bifunctional Antibody-fusion Molecule, in Combination With Sacituzumab Govitecan in Participants With Unresectable, Locally Advanced, or Metastatic Solid Tumors (START-002)

Inclusion Criteria: 1. Have measurable disease as per RECIST v1.1 criteria and documented by CT and/or MRI. Cutaneous or subcutaneous lesions must be measurable by calipers. 2. Tumor Type: * mTNBC (Safety Run-in and Cohort A): Progression or recurrence of locally advanced or metastatic TNBC * HR+/HER2- mBC (Safety Run-in and Cohort B): Progression or recurrence of locally advanced or metastatic HR+/HER2- breast cancer 3. Symptomatic central nervous system (CNS) metastases must have been treated, be asymptomatic for ≥ 14 days, and meet the following at the time of enrollment: No concurrent treatment for CNS disease (eg, surgery, radiation, corticosteroids \> 10 mg prednisone/day or equivalent); No concurrent leptomeningeal disease or cord compression. Exclusion Criteria: 1. History of known autoimmune disease with exceptions of: * Vitiligo * Psoriasis * Atopic dermatitis or other autoimmune skin condition not requiring systemic treatment * History of Graves' disease, now euthyroid for \> 4 weeks * Hypothyroidism managed by thyroid replacement * Alopecia * Arthritis managed without systemic therapy beyond oral nonsteroidal anti-inflammatory drugs * Adrenal insufficiency well-controlled on replacement therapy 2. Major surgery or traumatic injury within 8 weeks before first dose of study intervention 3. Unhealed wounds from surgery or injury 4. Clinically significant cardiovascular/vascular disease, gastrointestinal disorders, inflammatory processes, pulmonary compromises 5. Active viral, bacterial, or systemic fungal infection requiring parenteral treatment within 7 days prior to the initiation of study intervention. 6. Vaccination with any live virus vaccine within 4 weeks prior to the initiation of study intervention administration. Inactivated annual influenza vaccination is allowed. 7. Participants who are known to be human immunodeficiency virus positive or hepatitis B or C positive and have uncontrolled disease. 8. Second primary invasive malignancy not in remission for ≥ 1 year. Exceptions include non-melanoma locally advanced skin cancer, cervical carcinoma in situ, localized prostate cancer (Gleason score ≤ 7), resected melanoma in situ, or any malignancy considered to be indolent and never required systemic therapy, with the exception of indolent lymphomas. 9. Pregnant, likely to become pregnant, or lactating women (where pregnancy is defined as the state of a female after conception and until the termination of gestation) 10. Treatment with \>10 mg per day of prednisone (or equivalent) or other immune-suppressive drugs within 7 days prior to the initiation of study intervention. Exceptions may be made for participants who have had allergic reactions to iodinated contrast media. Steroids for topical, ophthalmic, inhaled, or nasal administration are allowed