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Arc is based on Chromium for anyone wondering. I believe part of the UI is written in Swift but the engine isn't.

I like Arc and use it (I actually never stopped using it after the first time I opened it), but I wish they weren't building off of Chrome.

Can user enhance it with extensions?

Yes

Arc is a fork of Chrome if I'm not mistaken

The rendering uses Chromium engine. But all of the UI is built in Swift. Both on macOS and Windows