For independent or specific songs not found in libraries, you must contact the owner—usually the artist or their publisher.
Pay an upfront fee (starting around $100 for smaller creators) to keep 100% of your video revenue.
Specifically designed for YouTubers to license mainstream, copyrighted music without getting a copyright strike.
Look for music under CC-BY (requires attribution) or CC0 (public domain) on sites like Free Music Archive . License tracks - YouTube Help