youtube-dl is not available for iPhone but there are some alternatives with similar functionality. The best iPhone alternative is Savemp3.cc, which is free. If that doesn't suit you, our users have ranked more than 100 alternatives to youtube-dl and five of them are available for iPhone so hopefully you can find a suitable replacement. Other interesting iPhone alternatives to youtube-dl are YourTube (Paid), Save Fb Video (Free), FastFrom.com (Free, Open Source) and Mp3converter (Free, Open Source).
An online free YouTube converter which allows you to convert videos and audios to MP4, MP3, M4A from YouTube, Facebook, Instagram, MixCloud, Dailymotion, VEVO, bilibili, niconico, etc. Supports HD MP4, 720p, 1080p video and 320kbps MP3.
Yourtube adds a fully integrated downloader to the YouTube app of your jailbroken iPhone or iPod Touch, so you can keep those great videos that you love to watch. Yourtube is available in Cydia (jailbroken devices only).
Mp3 converter is an amazing online video converting tool, which allows you to download your favourite music in any desired format. It provides you a variety of formats to choose from, such as Mp3, WebM, MPG, AVI, WMV, MKV, Mp4 and more.