Honestly, how race/ethnicity work in a SF universe is a direct reflection of the SF author's vision for the future.
In a future where the majority of human cultures are perfectly well assimilated with each other, everyone should be intermixed and "look Brazilian". An alien anthropologist could honestly describe humans as simply "Human" - no racial or ethnic qualifier. However, in order for all of humanity to become highly mixed, we have to actually mix with each other. Even on a highly integrated Earth there may be nations, cities within nations, or ghettoes within cities which are virtually mono-ethnic. In a racially-blended future, the few humans who are clearly identifiable as one race may be treated as outliers. Maybe as primitive weirdos, maybe as a highly valuable endangered species, maybe as a propaganda symbol for neo-nationalists. Maybe all of the above.
As humans leave the Earth and settle interplanetary/interstellar colonies, there is a high chance that some of those colonies will be nearly mono-ethnic. If there is enough inter-colony mobility then they may eventually blend into the "looks Brazilian" mixed race, but it is possible that some colonies are isolated enough to stay ethnically pure. In some fictional settings, such as Peter Hamilton's Nights Dawn Universe, many planets are intentionally designed as "ethnic streaming" colonies.
Even a mixed-race extraterrestrial colony could end up in a situation similar to the USA, where we have a lot of different races and a decent amount of interracial mixing, but there's still plenty of racial conflict. If you're the author writing the scenario, it all depends on how much you want to use race/ethnicity/nationality as a plot device for causing conflict. (Personally, I believe that racial/ethnic conflict will never go away)