The is a simple userscript allows you to get download YouTube videos without 3rd-party service.
You could take a look of the source code, everything is fetch from youtube directly.
And it also works on mobile browsers that support extensions, such as FireFox, Yandex and Kiwi. AdGuard Android's "extensions" is also supported.
Note: This script only supports Violentmonkey & Tampermonkey.
Why should I use this over youtube-dl?
If fact, youtube-dl is much more reliable than this script. If you are find with youtube-dl, just stick with it. But the point of this script is that you can easily download a YouTube video within a click, no need to open your terminal and enter youtube-dl command.