Mythology is defined as being a particular collection of myths shared in a specific area. They usually accompany a religious body of belief. Mythology is an oral tradition and has existed since the first civilizations with written history. These bodies of belief can encompass many different countries and regions with no regard to borders or ethnicity.
Mythology is based around the same basic question that science is, which is how do humans relate to the world around them. Where science uses observation and experimentation, mythology used stories and lessons. Each branch of mythology stems from people trying to understand their place in the world. The variety in Mythology shows that humans are variable creatures with many different ways of looking at life.
With the varied mythologies all being made in relative close proximity, the borders are naturally hard to pin down. An easy way to know where an early set of myths was based is to identify the language of the primary text, as early diffusion of culture was very hard when people couldn't understand each other. The main way a new belief became popular is when a force physically took over an area and settled in the region. Good examples would be the anglo-saxon invasion into Britain in 450ad, where they converted the local britons to paganism
While most mythologies grew from a seemingly original group of myths, others simply renamed gods from an older civilization and worshipped them as their own. The biggest name in additive mythology is Roman Myths. The romans trace their origins to Troy which is geographically close to Greece, located just east across the Aegean. The Roman Gods have largely original names, but are almost carbon copies of the Greek Pantheon. Another early example of additive mythology is Persia, but even they had their own original gods that they added to when they conquered other peoples.
It should be stated that mythologies and religions are not the same same thing. Religion is a personal or institutional set of religious attitudes, beliefs, and practices (Merriam Webster) Whereas Mythology is a semi-organized group of stories that usually accompany a religion and contain the main themes and characters of importance to that religion or group of people.