Totally free, 5KPlayer from DearMob is a media-playing utility that is a lot more than a downloader—but it's got a heck of a downloader integrated. Promising no viruses, ads, or plug-in requirements is a good start. It is, sadly, one of the few I tested that asks for a registration of your name and email—you have to do that to get the full download function across 300+ sites. You may still use it if you don't register; I didn't and was able to download 4K vids from YouTube.
Step 1: Free download YouTube video downloader on Mac iMac Pro, Macbook Air/Pro or Mac Mini and install it, copy the URL of the video page. find a video you'd like to save offline, then paste the URL and click "analyze" to load the YouTube video details. Or click "paste & analyze" button to automatically detect the YouTube video opened in browse.