A Pharmacy and Therapeutics (P&T) committee is the clinical panel behind a formulary. Convened by a PBM, an insurer, or a large employer's benefits consultant, it is typically made up of physicians, pharmacists, and other clinical experts who review the evidence for each drug (efficacy, safety, and how it compares with alternatives). They decide which drugs are clinically required, acceptable, or unacceptable, which ones are clinically equivalent, and what restrictions such as prior authorization or step therapy should apply.
The commercial choices usually come next, inside that clinical framework. When the committee has judged several drugs equivalent, a PBM typically negotiates rebates among them and uses the results to decide which one is preferred, which tier each lands on, and which is left off, as the NAIC's PBM white paper (external link) describes. So a GLP-1 decision like "cover tirzepatide but not semaglutide" usually reflects the rebate deal as much as the clinical review. Formularies are usually revisited at least annually, which is why coverage can change from one plan year to the next. See how to determine if your formulary covers GLP-1 medications.