To me (and, I suspect, to Gitlab), "underrepresented" means "present in lower proportions than in the general population".
South Africa, to use one of your examples, has a majority Black African population (80%), but business management is overwhelmingly White[1] (67%).
That means Black African people are simultaneously the majority of the country and also underrepresented minorities that struggle with systemic, institutional racism.
In what countries are over-represented groups mistreated by systems or institutions?