Anything irrelevant to plot or story building, such as nationality, are usually left unspecified. Readers often project their own identity into characters, and some people seem to find it odd that they do not encounter traits that differ from their own.
Ethnicity is sometimes incorporated, that is, some distinctions would be necessary if there was a documentary manga about a match in an Olympic Games played by teams from multiple parts of the world, and in that case an American players might be given smaller eyes or extra wrinkles in face, or African players might be colored darker than other characters, Chinese players could be drawn with slightly different shapes of chins, etc.
But the default is unspecified or an averaged, most simplified shapes and forms that the author uses in their own cognition.
Ethnicity is sometimes incorporated, that is, some distinctions would be necessary if there was a documentary manga about a match in an Olympic Games played by teams from multiple parts of the world, and in that case an American players might be given smaller eyes or extra wrinkles in face, or African players might be colored darker than other characters, Chinese players could be drawn with slightly different shapes of chins, etc.
But the default is unspecified or an averaged, most simplified shapes and forms that the author uses in their own cognition.