Downloading web video

• Download and install a Youtube download application, I use: iTubeDownloader

in iTubeDownloader, search for the video you want to download then click Download button

• open the downloaded video in QuickTime Player and trim the video to length: go to Edit > Trim

drag the trim handles to the new start and end points

click Trim Button when completed

• save the file: go to File > Save

• Drag and drop the video file on to a slide in Keynote

Please bear in mind some videos are prevented from being downloaded and that all videos in Youtube are copyrighted and may have restrictions on there use.

If you're comfortable using Terminal, I would recommend using youtube-dl: https://youtube-dl.org

It supports YouTube, Vimeo, and other similar sites.