I see the Opera extension is at https://addons.opera.com/en/extensions/details/youtube-downloader-udl-helper/.
For Chrome, it uses Tampermonkey and this huge user js script:
https://unidownloader.com/src/usrjs/downloader.user.js
It has all kinds of modules and seems to use a web assembly version of ffmpeg.
At, https://unidownloader.com/en, there's the client app. I assume the helper extension communicates with it.
As to whether it's safe or not, not sure.