Yes, it's does ... I use DDG. Yes, addons work as per Chrome on any other OS.
In fact, when he logs in (so long as he's syncing everything already), it will all sync to his Chromebook, so extensions, settings, password, etc. Since he's clearly into tinkering with extensions for privacy, he can also setup the networking stack as well, particularly DNS to point to, say, 1.1.1.1 (Cloudflare) or 9.9.9.9 (Quad9) for DNS privacy.
Again, if folks want a preview of what using a Chromebook might be like without actually buying one, this distro can be written to a USB key and booted from:
https://www.neverware.com/ < ChromeOS (from Google) does have more bells and whistles, like the Play Store, but this distro is very much the core of ChromeOS.
Just don't install it as it will wipe the computer! Unless you want to ...