How are you creating your videos? You MUST be working within YouTube Studio to access all of the tracks that can be used with videos on YouTube. There are well over 100,000 tracks available but they CAN'T be downloaded or remixed.
YouTube Audio Library is a separate facility, with far, far fewer tracks but those that are available can be downloaded for use anywhere at all. (i.e. not only on YouTube).
Perhaps you're confusing the two?