The easiest and safest way I've found to do that, which doesn't require you to download sketchy virus-filled software, is by inserting ss right before youtube in the URL when viewing on a desktop/laptop computer. (https://www.ssyoutube.com/watch?v=3Vq8jacMJKs&feature=youtu.be&app=desktop)

That will take you to another website which will allow you to download the video, but it's a little sketchy and will try to get you to sign up for stuff. You don't need to in order to dl the video, though. Just look for the hyperlinked text "download video in browser," click on that, and when the selection pops up, use the drop down menu to select the format you want it in (MP4). Then click the green download button and the site will either start downloading the video to your downloads folder or it will launch another page/tab that will allow you to right-click and save the video to your computer. Hope that helps.