If you purchase an iPhone outside your home country, it will have NO warranty in your country. And if you add an AppleCare+ plan to the phone in the country at purchase and try to use out in another country, it won't be valid. If you still plan to buy your phone in US, do NOT buy an AppleCare+ plan with it.
You should NOT buy a phone in the US if you don't live in the US. iPhone's sold in the US are dual eSIM only. The warranty won't be valid in your country and any savings you think you'll get by buying the phone outside your country will evaporate and then some, if you ever need warranty support.
Finally, if you are still not dissuaded from buying your phone in the US, buy it ONLY from Apple. Do NOT buy it at a BestBuy or carrier store as ONLY Apple sells fully unlocked phones. Buying it anywhere else and you'll find you can't even connect to a carrier in Brazil when you get home. Again, DON'T buy your phone in the US if you live in Brazil. Just DON'T do it.