You are requesting advice on how to illegally download content that pertains to YouTube and this is a clear violation of their terms of service which you agreed to as a user.
It is against YouTube's terms of service to download clips or videos in any way shape or form, even for offline usage or private usage. That's why after many years they now created YouTube RED which gives several additional privileges.
Every YouTube video is uploaded with a Standard YouTube license which means that the video is only available as it is, through the platform. Now, you can try checking the creative commons section.
The Creative Commons license means that you can use the content with the online video editor (www.youtube.com/editor) to edit parts of the video to create a new video. Even if you don't want to edit the video, this option allows you to download the video to your computer or cellphone and re-upload to other channels, so check if the creator has such videos available under the Creative Commons editor. If not then no, you can't download his content for your own private usage.