Some countries could be part of larger regions, other countries could be divided into separate regions. For example, Saudi Arabia is part of the Middle East. However, in a country like the United States, there are multiple regions, like the West, South, Southeast, Midwest, Northeast, and Great Plains.
No, of course not. The larger the country the more likely it will have multiple cultures. Also many modern cities are large enough that individual neighborhoods whose inhabitants are largely from one group of immigrants have a different culture in each of these neighborhoods.
Teotihuacan began as a religious center in the Mexican Highlands around the first century CE. It became the largest and most populated center in the pre-Columbian Americas. that is all I know about it
