@hevov No, It's using the Webview Implemantion of the OS, which is different than a OS based on chromium. Since it's function differently, probaly is based on Chromium, but is more like a fork of it. Windows has Edge Webview2 (Which isn't that much related to it), Android has Android Webview or Chrome if Webview isn't installed, you can also install alternative Webviews on Android. About iOS and MacOS, I don't rly know. They have probaly smth with webkit.
@grafcube @opensource @privacy I will make some comments to it, when I finished reading. I just want to say u already got some points wrong, Brave plans to continue supporting MV2 too, same for Vivaldi as far ik. Also DuckDuckGo's Browser are not chromium based too, they use the Systems Webview.
Edit: removed the info that Brave will not support WEI, since it got later mentioned in the blog post