Race itself is almost impossible to pin down, even if you begin throwing genetics around, the "frontiers" of a race against another is always blurry, there's no line, it ends up going to a kind of "Wer Jude ist, bestimme ich" - "I determine who is a Jew"
Race is defined by those who wrap their minds around it the most, and it is then imposed on those who were too busy living a life to give that kind of arbitrary shit much thought. The most obvious racial definition is visual, is someone browner than you, or whiter than you. Beyond that, we go into fun land deluxe, measuring facial proportions, Mick Jagger for example, is a black guy with white skin [insert laugh track]
Other definitions are linguistics, and - if it's even possible beyond just... assertion/accusation - what heritage one might have, and of course, religion-on-on-on (dramatic echo)
In the very end, the only ones who benefit from the definition of people into races are the ones who typically do benefit from divide-and-conquer type of policies. We are not a people, clearly, we are many fragmented peoples, we are weak and insecure, and we are always threatened by them other peoples, those guys, and those, and those *points, points and points*
we look towards our authorities, up on their thrones, and blink our Bambi eyes, we BEG them to retain power, we offer them our hard labour and our obedience, if only they will keep us safe.
They go "hmmmmmmmmmmmm!"
Our lips quiver.
They go "okay then"