Ensure you allow "Untrusted Shortcuts" in your iPhone settings.
When watching a video in the YouTube app, tap Share, then More, and select your shortcut. Choose the "Save as Audio" option. download youtube audio high quality iphone
Open Safari and navigate to a reputable YouTube to MP3 converter like Y2Mate or 320YouTube. Paste the link into the search bar. Select the highest bitrate available (320kbps is ideal). Tap Download and confirm the prompt in Safari. Ensure you allow "Untrusted Shortcuts" in your iPhone