For a true audio-only experience where you don't store bulky video files, the YouTube Music app is the primary tool included with your Premium subscription. Get YouTube Premium
YouTube Premium offers a direct, official way to enjoy audio offline, though the method differs depending on whether you are using the main YouTube app or the dedicated YouTube Music app. While the main YouTube app primarily downloads full videos, its integration with provides a seamless "audio-only" download experience. Official Methods for Audio-Only Downloads youtube premium download audio only