The active pharmaceutical ingredient (API) is the actual drug molecule in a medication — semaglutide or tirzepatide, as opposed to the water, buffers, and preservatives it's dissolved in. Brand manufacturers make their own API under FDA manufacturing rules; a compounding pharmacy instead buys API in bulk from a supplier and prepares it into a finished product.
Where the API comes from is the single biggest quality question for a compounded GLP-1. Legitimate pharmacies source from FDA-registered facilities and receive a certificate of analysis for each lot, and the FDA has said that semaglutide salt forms (sodium or acetate) are different active ingredients from the base semaglutide in approved drugs, with no lawful basis for compounding. Asking a pharmacy where its API comes from is a fair question; see how compounded GLP-1 medications are made.