A Study of Sacituzumab Tirumotecan (MK-2870) as a Single Agent and in Combination With Pembrolizumab (MK-3475) Versus Treatment of Physician's Choice in Participants With HR+/HER2- Unresectable Locally Advanced or Metastatic Breast Cancer (MK-2870-010)

Inclusion Criteria:

* Has unresectable locally advanced or metastatic centrally-confirmed hormone receptor positive (HR+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer
* Has radiographic disease progression on one or more lines of endocrine therapy for unresectable locally advanced/metastatic HR+/HER2- breast cancer, with one in combination with a CDK4/6 inhibitor
* Is a chemotherapy candidate
* Has an eastern cooperative oncology group (ECOG) performance status of 0 to 1 assessed within 7 days before randomization
* Has adequate organ function
* Human immunodeficiency virus (HIV)-infected participants must have well controlled HIV on antiretroviral therapy
* Participants who are Hepatitis B surface antigen (HBsAg) positive are eligible if they have received HBV antiviral therapy for at least 4 weeks, and have undetectable HBV viral load
* Participants with a history of Hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable

Exclusion Criteria:

* Has breast cancer amenable to treatment with curative intent
* Has experienced an early recurrence (\<6 months after completing adjuvant/neoadjuvant chemotherapy) and therefore is eligible to receive second-line (2L) treatment
* Has symptomatic advanced/metastatic visceral spread at risk of rapidly evolving into life-threatening complications
* Has received prior chemotherapy for unresectable locally advanced or metastatic breast cancer
* Active autoimmune disease that has required systemic treatment in the past 2 years
* History of (noninfectious) pneumonitis/interstitial lung disease that requires steroids, or has current pneumonitis/interstitial lung disease
* Has an active infection requiring systemic therapy