• Participant has signed the informed consent before all study specific activities are conducted.

• Women or men aged ≥18 years (or the minimum age of consent in accordance with the local law), at the time of informed consent signature. Female participants may be of any menopausal status.

‣ Postmenopausal status is defined as follows or in accordance with local regulations:

⁃ Age ≥60 years or

• Age \<60 years and amenorrhea for 12 or more months (without an alternative cause) and follicle-stimulating hormone value and an estradiol level within the postmenopausal range per local laboratory reference or

• Documentation of bilateral oophorectomy, at least 1 month before first dose of trial therapy.

⁃ Premenopausal and perimenopausal women (who do not fit postmenopausal criteria) and men must be receiving a luteinizing hormone-releasing hormone (LHRH) agonist and must be initiated at least 3 weeks (4 depending on local label) before the start of trial therapy and are planning to continue LHRH agonist treatment during the study treatment.

• Histopathological or cytological confirmed ER+, HER2-, breast cancer, per local laboratory, as per the American Society of Clinical Oncology/College of American Pathologists guidelines. Note: In the context of this trial, ER status will be considered positive if ≥10% of tumor cells demonstrate positive nuclear staining by immunohistochemistry, with or without progesterone positivity.

• Documented radiological disease progression during or after the most recent therapy.

• At least 1 measurable lesion as per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1). Tumor lesions previously irradiated or subjected to any locoregional treatment will only be considered measurable if there is clear, documented progression at the treated site. For participants with bone only disease, lesions: must be lytic or mixed (lytic + blastic / sclerotic), confirmed and accurately assessed by computed tomography or magnetic resonance imaging, and must have an identifiable soft tissue component meeting the definition of measurability per RECIST v1.1. Note: participants with blastic / sclerotic bone lesions only are not eligible.

• Eastern Cooperative Oncology Group performance status of 0 or 1.

• Participant has adequate bone marrow and organ function, as defined by the following laboratory values:

∙ Absolute neutrophil count ≥1.5 × 10\^9/liter (L)

‣ Platelets ≥100 × 10\^9/L

‣ Hemoglobin ≥9.0 grams/deciliter (g/dL)

‣ Creatinine is ≤ 1.5 x upper limit of normal (ULN) or if creatinine is \> 1.5 x ULN, then creatinine clearance must be ≥50 milliliters/minute based on the Cockcroft-Gault formula. Note: C-G formula:

⁃ Creatinine clearance (male) = (\[140-age in years\] × weight in kilograms \[kg\])/ (\[serum creatinine in milligrams/deciliter (mg/dL)\] × 72)

• Creatinine clearance (female) = (0.85 × \[140-age in years\] × weight in kg)/ (\[serum creatinine in mg/dL\] × 72)

• f. Serum albumin ≥3.0 g/dL (≥30 g/L)

• g. In absence of liver metastases, alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 3.0 × ULN. If the participant has liver metastases, ALT and AST ≤ 5 × ULN

• h. Total serum bilirubin \<1.5 × ULN except for participants with Gilbert's syndrome who may be included if the total serum bilirubin is ≤3.0 × ULN or direct bilirubin ≤ 1.5 × ULN.

‣ Additional Criteria for the Alpelisib Combination (Phase 1b and Arm A): In general, the prescription information of the respective combination drug should be consulted for instructions/restrictions with respect to interactions with concomitant medications.

• Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) mutation by local laboratory assessment.

• One or up to two prior hormonal therapies in the advanced or metastatic setting, one of which was in combination with a cyclin-dependent kinase targeting enzymes CDK4 and CDK6 (CDK4/6) inhibitor.

‣ Additional Criteria for the Everolimus Combination (Phase 1b and Arm B), the Abemaciclib Combination (Arm C), the Ribociclib Combination (Phase 1b and Arm C), and the Palbociclib Combination (Phase 1b): One or up to two prior hormonal therapies in the advanced or metastatic setting, one of which was in combination with a CDK4/6 inhibitor.

‣ Additional Criteria for the Palbociclib Combination (Arm D), the Abemaciclib Combination (Arm D), and the Ribociclib Combination (Arm D): One or up to two prior hormonal therapies in the advanced or metastatic setting.

‣ Additional Criteria for Capivasertib Combination (Phase 1b and Arm E): Recruitment in this combination will occur only in countries where capivasertib is locally approved and available.

• PIK3CA/AKT1/PTEN-alteration as detected by an FDA and/or locally approved test (local result).

• One or up to two prior hormonal therapies in the advanced or metastatic setting or participants who have radiological evidence of breast cancer recurrence or progression within 12 months from the end of adjuvant treatment with endocrine therapy, as these participants are considered as first line relapsed participants. Prior CDK4/6i treatment is allowed but not required.