The confusion is yours: citizenship and nationality are equivalent terms. You might be thinking of residency status, which gives you the right to live and access to all of the welfare system, but does not make you a citizen, so you don’t have the right, e.g., to vote for national elections nor gives you a Portuguese passport.

Unfortunately Brazilians have a kind of automatic citizenship on arrival from an agreement between Portugal and Brazil.

Either you are grossly simplifying their extended visa process or you are utterly misinformed.

There is no “automatic citizenship” for Brazilians. The only path to citizenship in Portugal is by bloodline and even that has become harder to get: it used to be that grandchildren of Portuguese citizens could start the process to get citizenship directly, and some good ~15 years ago it changed that only the direct descendants can do it.