I've seen it discussed before that the ideas of nations and race are relatively recent developments in the grand scheme of human history. As I understand it, even lines on a map are problematic in capturing how polities of the past actually operated.
Now, recognizing that something like this will vary wildly over time and space, how might a member of a specific culture group such as the Greeks or Germanic tribes consider themselves relative to their neighbours? Would they see it through physical differences such as appearance or clothing, or through a political lense like the Lords or King they served? Would it be based on language? Food?
As an example, how might a medieval Englishman or classical Greek describe the difference between them and their neighbours? Or am I entirely off and it is so varied as to be an unanswerable question altogether?
I love this question, and it gets at one of the fundamental questions for the entire discipline of historical anthropology: how did people of the past see themselves in relation to other societies and cultures?
The short answer is that there are no easy answers. One argument is that identity is always contingent upon social ties and institutions. For example, it's become fairly well accepted among mainstream Early-modern Ottomanists that identity for inhabitants of that empire was more closely tied to religion than to language or even culture prior to the 19th century (see Maria Todorova, "Imagining the Balkans" and Tijana Krstić, "Contested Conversions to Islam: Narratives of Religious Change in the Early Modern Ottoman Empire"). In mixed societies, institutions like church and tax regimes probably had a much more impactful and enduring role in people's lives than ethnicity or language, and we usually underestimate just how mixed people were in premodernity.
There's also a strong argument that people have habitually constructed identity according to their contemporary civilizational or ethnic hierarchies. I'm not deeply knowledgeable about ancient history, but the concept of dividing the world into "civilized" and "barbarian" is an old one. According to this argument, as Europeans began to encounter radically different societies in the sixteenth century, they sought to fit them into preexisting hierarchies of "otherness," leading to Orientalism and scientific racism (David Abulafia, "The Discovery of Mankind", Edward Said, and Larry Wolff). Early-enlightenment thinking about human difference is fundamentally an attempt to identify the scientific principles governing racial and ethnic difference, with a more or less explicit bigotry about who sits on top of the civilizational pile. You even see seventeenth century natural philosophers explaining racial difference as being caused by the fact that the middle latitudes are closer to the sun than the poles (and the Poles too, ha) (Fontanelle, Plurality of Worlds).
There's also a lot to be said about ethnic stereotypes, particularly in the greater Mediterranean world. At least by the seventeenth century and probably long before, national differences (in the premodern sense of the term) seem to have been constructed and viewed primarily according to emotional or cultural stereotypes. We still know many of these: the English love to "politick", the Spanish are proud but lazy, the Dutch are mercantile and serious, the Germans are "cold" but loyal. Various "Tables of Nations" were written or printed in the seventeenth and eighteenth centuries depicting the supposed qualities of various ethno-national types. These were often expanded to include Scandinavians, Central and Eastern European nations, and Turks/Greeks. It's telling that even today we tend to construct our sense of identity and impressions of other cultures against or according to reductive stereotypes.
There's one final argument to note: the functional role of constructing ethnic otherness for the purposes of political exclusion and domination. You might call it the Postmodern interpretation; the observation that negative depictions of the characteristics of other cultures and ethnicities often go hand in hand with efforts to control, enslave, or kill. We see this about the Jews and Romani in Europe and beyond, Africans in the New World, Eastern Europeans under the Habsburgs (and Third Reich!), Armenians and Albanians in the Ottoman Empire at various points, and the Irish in Britain.
Hope this is a good starting point. As you can see, you have a wide variety of methodologies and perspectives to choose from.