2 added 485 characters in body
source | link

The general guideline is that apps must be legal and downloading youtube videos is not allowed according to their terms of service and is therefore most likely not legal.

And directly from apple:

5.2.3 Audio/Video Downloading: Apps should not facilitate illegal file sharing or include the ability to save, convert, or download media from third party sources (e.g. Apple Music, YouTube, SoundCloud, Vimeo, etc.) without explicit authorization from those sources. Streaming of audio/video content may also violate Terms of Use, so be sure to check before your app accesses those services. Documentation must be provided upon request.

developer.apple.com/app-store/review/guidelines/#objectionable-content

The general guideline is that apps must be legal and downloading youtube videos is not allowed according to their terms of service and is therefore most likely not legal.

The general guideline is that apps must be legal and downloading youtube videos is not allowed according to their terms of service and is therefore most likely not legal.

And directly from apple:

5.2.3 Audio/Video Downloading: Apps should not facilitate illegal file sharing or include the ability to save, convert, or download media from third party sources (e.g. Apple Music, YouTube, SoundCloud, Vimeo, etc.) without explicit authorization from those sources. Streaming of audio/video content may also violate Terms of Use, so be sure to check before your app accesses those services. Documentation must be provided upon request.

developer.apple.com/app-store/review/guidelines/#objectionable-content

1
source | link

The general guideline is that apps must be legal and downloading youtube videos is not allowed according to their terms of service and is therefore most likely not legal.