You can locate the YouTube Audio Library in the Creator Studio section of your YouTube account or access it directly via this link.
There are many high quality music tracks available for you to use on your videos for free, even if your videos are monetized.
Please note that some audio files in the YouTube Audio Library are marked with a different license and require attribution. The rest require no attribution.
YouTube doesn’t speak to whether or not you can use these tracks elsewhere, and that’s probably because it varies by track and license. To keep life simple, I only use these tracks on YouTube, but if you want to do your homework you can track down the original author of a track and see if they have released it elsewhere and clarified the licensing.
Note that you may have to be logged into YouTube for the link above to access the Audio Library to work.