What was even the point of all this roundabout engineering and the time and manpower to do so? What a waste. This seems more like an acquihire than actually about IP.
I never really got where the innovation was in Arc, and never got a chance to see or try Dia, but the interview at least gave me some empathy for what they were going for.
Arc breaks the traditional paradigm of "bookmarks", using "Spaces" and "Pins" instead.
"Pins" are the closest to bookmarks. They appear at the top of the sidebar, and can be organized into folders. They differ from bookmarks in that they are pretty much native tabs that are unloaded until you open them.
"Spaces" are sets of tabs, both pinned and normal. You can associate a space with a specific profile, and each profile has separate cookies and such - but you don't have to.
From a usage perspective, pinned tabs instead of bookmarks mean that I can press Cmd+t and enter a URL, a search query, or the name of a pinned tab. It's smart enough to choose the correct one which means I don't have to think about it. That's handy, and was unique when I adopted it, but I'm sure it's either been implemented in other browsers or can be easily enough.
Spaces are _very_ handy, though. Right now, I have eight spaces. One "default" that I use normally, five for various projects I'm actively working on, one for a long-running project that requires me to log into a different Zoom account (so that one has its own profile, to prevent my accidentally being logged into the wrong Zoom account when joining a meeting), and one for personal stuff.
I have it on my list to look for an alternative and migrate, but it's still working for my needs. I'm going to miss it, though.
I worked with a PM that absolutely loved it and insisted on using it. When he showed me it, all I could think of was "this is what my Firefox looked like before they killed XUL extensions".