No longer any easy way to download videos.
Downloading is against the terms of service:
You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content. You shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content.
I cannot find a download link on videos that I have marked as CC-BY or other CC-BY videos that I have come across.
In the past, the YouTube video editor allowed the reuse of creative commons videos. At one time there was also an option to save videos for remixing.