Start by searching for "central place theory" and it will lead you down more rabbit holes than you ever considered. The basic notion is that goods and services have a radius in which it's profitable to exist. Outside of that radius, another one will appear, leading to (under ideal circumstances of a uniform flat plane populated by perfectly rational producers and consumers) a hierarchy of settlements laid out in hexagonal grid. Real-world constraints such as mountains, rivers, and politics will distort that perfect grid, with sufficient distortion resulting in additional features (e.g. a ferry on a river or a settlement on each side of a mountain pass) or removal of features (that town done got ate by the dragon, that's why). Towns have a tendency to be about a day's round-trip travel away from each other, with various support features at half-intervals. The kind of goods and modes of transport will also play into it and changes in transportation modes will disrupt existing networks (e.g. the advent of the automobile and interstate highway system in the USA badly disrupted many areas in ways still not recovered). There's a lovely article (dissertation?) on why the counties and states are different sizes that you'll come across: read it!