Inclusion Criteria:  Pre-Screening Phase Biopsy-proven ER-positive, HER2-negative breast cancer. ER-positivity and PR-positivity are defined as ≥1% cells staining positive by immunohistochemistry. HER2-negativity is defined by IHC or FISH, per ASCO-CAP 2018 guidelines. Breast tumor must be intact and tumor size must be ≥ 1 cm as measured by ultrasound, mammogram, MRI, or clinical exam. If tumor is locally recurrent, it must be in the breast (not skin, node, or chest wall recurrence). Ki67 may or may not have been done locally but if done locally, must be ≥ 5%. Any nodal status is allowed, as M0 or M1 disease. Women or men, age ≥ 18 years old. Able to swallow and retain oral medication. Performance status 0 to 1 (by Eastern Cooperative Oncology Group [ECOG] scale). Ability to understand and the willingness to sign a written informed consent document.  Treatment Phase  Breast tumor classifies as relevant integrative subtype per tumor sequencing performed and analyzed by central laboratory. Breast tumor Ki67 score ≥ 10% as assessed by central laboratory. Each investigational agent specific inclusion criteria can be found in the agent-specific appendix  Exclusion Criteria:  Pregnant woman, as confirmed by positive serum hCG test prior to initiating study treatment. Nursing (lactating) woman also not allowed. Prior breast cancer-directed therapy (surgery, radiation, chemotherapy, or endocrine therapy) is not allowed, with the exception of people with in-breast recurrences. People with in-breast recurrences cannot have had breast cancer-directed therapy (radiation, chemotherapy, or endocrine therapy; surgery is acceptable) within the 6 months prior to signing the pre-screening consent. Pre-endocrine therapy for breast cancer risk reduction is allowed. Known hypersensitivity to study agent (IP) or standard endocrine therapy drug, or to any of the excipients of study agent (IP) or standard endocrine therapy drug. Impairment of gastrointestinal function or gastrointestinal disease that may significant alter the absorption of the study drugs (e.g. ulcerative diseases, uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome, or small bowel resection) based on investigator discretion Each study agent specific exclusion criteria can be found in the agent-specific appendix