I'm hoping for a more specific answer, but here's what the airline, Azul Linhas Aereas Brasileiras, replied:
As Brazil and Argentina have a territory agreement in South America, you will just make a short connection in Belo Horizonte, with no need of new customs.
There was a ground agent at the end of the jetway upon arrival. I asked what to do for my international connection and was told to wait with there with them. They then started calling out "Orlando" (the apparent only international transit destination) to try and find other transit passengers. They located two other passengers and brought us up a staircase and unlocked the door into the international departure lounge. Very straightforward and only took a couple minutes.