At EZE, terminals A, B, and C are connected airside, although the check in areas are distinct. It's really one building and you can walk, although it can be a long walk.

I don't know how the few domestic flights at EZE are handled. What I've observed is that it all seems international, with passport control and security before one can go airside. Upon arrival, you're funneled directly to immigration, bag claim, and customs (where all luggage is X-rayed). I've never seen any path upon arrival that could lead to lounges.