If you're using Chrome, like many people do, you're not allowed to have an extension that can download YouTube videos. Because it's bad, and Google doesn't like it. But anti-virus people Avast have their own browser and that means they don't have to play by Google's rules. So Avast Secure Browser actually has a YouTube downloading tool built-in, among other things.
You may remember Avast Secure Browser as its former name Safe Zone, and if you used the company's anti-virus you may have found that it had a nasty habit of seemingly installing itself without asking. the Chromium-based browser also has a bunch of helpful features, including a bank mode for added security, a VPN, HTTPS encryption, a password manager, anti-tracking tools, and other things theprivacy-conscious people out there would have to add to other popular browsers.
Oh and there's the video downloader which is capable of ripping videos from all sorts of different video sites like YouTube and Vimeo. It's a bit of an odd addition, but it's there and might entice some people into giving it a try. According to TechRadar it's quite easy to use, automatically detecting videos on any given page and offering users the option of downloading them in low, medium, or high quality. It also lets you just rip the audio, which seems like an odd choice since it could easily be used to enable music piracy. Then again, other companies have got away with offering such tools in the past. If it's not in Google's ecosystem, there isn't a whole lot they can do about it except take legal action.
Naturally if you're going to give it a go our professional advice is don't use it for anything illegal or infringes copyright. Not that we, or anyone else, has the power to stop you.