If I understood your question the right way, you were asking who would be the recipients of these interest payments that are presented in the main estimate documents.
The line item for interest on mature debt, which, as you point out, is approximately $15 billion, is a reflection of interest we will pay out on un-matured debt. That would be Government of Canada bonds, treasury bills, real return bonds, and other market-based instruments. The ultimate recipients of those interest payments would be holders of those instruments and securities.