It's based on Chromium, wich is C++. In terms of total code, I'd wager it's around 99% C++.
I think he means GUI not engine as they are indeed just using chromium engine as browser.
Yes. Ignoring the engine there’s a lot of swift in how they build the ui components and sync tabs etc. they’ve built something similar to SwiftUI but it used winui/wpf (or whatever it’s called these days ) underneath.
The Arc browser borrows very little UI or tab/account syncing code from chromium. The non-V8+Blink bits are almost entirely written in swift as far as I know.
This means it probably has the most Windows Swift code of any app out there and that’s why I mentioned it.