Q: How do I get a youtube video onto my MacBook Pro and then into keynote to imbed the youtube video into my presentation

If you have obtained permission to download the specific file from Youtube, and the settings allow downloading that particular file, use a Youtube download app to download the file on to the Mac, (google search for applications) then drag and drop the file on to a slide.

If you'll have an internet connection during your presentation you don't need to download the YouTube video. There's a free app called LiveSlides which will embed the YouTube video into a Keynote slide and stream it during your presentation. The how-to article at: https://www.liveslides.com/how-to-insert-youtube-into-keynote will show you how. Caveat: You do need to be able to install add-ins and you'll need to make sure you'll have internet access during your presentation.

The best suggestion is a dedicated Youtube download application. Pay attention to the copying and distribution restrictions as has been suggested.

Second place is to get the original Youtube video download location

1. Select the Youtube video in Safari, and right-click (two-finger tap) on the video to get the current video URL. Copy that to the clipboard.
2. Launch the free VLC media player
   1. Open Network…
      1. Paste the URL from the clipboard into the VLC URL field, and click open.
      2. The video will open in window and start playing. Pause or stop it. you will see an equivalence (≡) symbol on an adjacent button. Click that.
      3. A playlist window will appear with the current video highlighted. Right-click (two-finger tap) to choose Media Information.
      4. Copy and paste the URL in the Location field into Safari. This is the real source video.
      5. Right-click (two-finger tap) on the video in Safari, and choose Download video. A videoplayback.mp4 will appear on your Desktop. Drag/Drop it into Keynote.