All of them do one basic thing: you give them a YouTube URL (or other video site if supported), they parse it, give you a choice of what size download you want, and provide a link to said download. All are free unless noted and work across all platforms.
Free; Opera, Safari, Chrome, Firefox, IE, Chromium-based browsers
This downloader tool is on all platforms, with an installer for Chrome that skips the Chrome Web Store. It puts a "download this video" menu right on any YouTube page you load, with all the format options on display.
Free; Firefox, Chrome
InsTube – Free Video and Music Downloader just for Android
source: https://www.pcmag.com/news/331905/how-to-download-youtube-videos / By Eric Griffith/ June 4, 2019