Fulvestrant 500 mg intramuscularly (IM) on Days 1 and 15 of Cycle 1 and on Day 1 of each subsequent 28-day cycle until PD or unacceptable toxicity.

If chosen by the investigator as the CDK4/6i, participants will receive abemaciclib 150 mg PO twice per day (BID) on Days 1-28 of each 28-day cycle until PD or unacceptable toxicity.

If chosen by the investigator as the CDK4/6i, participants will receive palbociclib 125 mg PO QD on Days 1-21 of each 28-day cycle until PD or unacceptable toxicity.

If chosen by the investigator as the CDK4/6i, participants will receive ribociclib 600 mg PO QD on Days 1-21 of each 28-day cycle until PD or unacceptable toxicity.

Only pre/perimenopausal female participants and male participants will receive a luteinizing hormone-releasing hormone (LHRH) agonist locally approved for use in breast cancer on Day 1 of each 28-day treatment cycle.

F1LCDx is a next-generation sequencing (NGS)-based in vitro diagnostic test that detects and analyses genomic alterations in circulating cell-free DNA (cfDNA) isolated from plasma derived from the anti-coagulated peripheral whole blood of cancer patients. It will be used to determine the eligibility of participants requiring confirmation of ESR1 mutation status (mutation detected [ESR1m] vs. no mutation detected [ESR1nmd]).

The cORR is defined as the percentage of participants with a complete response (CR) or partial response (PR) on two consecutive occasions at least 4 weeks apart, as determined by the investigator according to RECIST v1.1. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

DOR is defined as the time from the first occurrence of a documented objective response to PD, as determined by the investigator according to RECIST v1.1, or death from any cause (whichever occurs first). It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

The CBR is defined as the percentage of participants with stable disease for at least (≥)24 weeks or a CR or PR, as determined by the investigator according to RECIST v1.1. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

Time to chemotherapy is defined as the time from randomization until the start date of the first chemotherapy or death from any cause (whichever occurs first). It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

TTCD in pain severity is defined as the time from randomization to the first documentation of ≥2-point increase from baseline on the "worst pain" item score of the Brief Pain Inventory-Short Form (BPI-SF). It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

TTCD in pain presence and interference is defined as the time from randomization to the first documentation of ≥10-point increase in pain score, as determined using the European Organisation for Research and Treatment of Cancer Quality of Life-Core 30 (EORTC QLQ-C30) questionnaire. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

TTCD in physical functioning (PF) is defined as the time from randomization to the first documentation of ≥10-point decrease in PF score, as determined using the EORTC QLQ-C30 questionnaire. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

TTCD in role functioning (RF) is defined as the time from randomization to the first documentation of ≥10-point decrease in RF score, as determined using the EORTC QLQ-C30 questionnaire. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

TTCD in in Global Health Status/Quality of Life (GHS/QoL) is defined as the time from randomization to the first documentation of ≥10-point decrease in GHS/QoL score, as determined using the EORTC QLQ-C30 questionnaire. It will be analyzed in the FAS and in the ESR1m and ESR1nmd subgroups.

Incidence will be reported as the number of participants with at least one adverse event, with severity determined according to the National Cancer Institute Common Terminology Criteria for Adverse Events, version 5 (NCI CTCAE v5.0).

Vital signs include respiratory rate, pulse rate, systolic and diastolic blood pressure, and temperature.