The short version is that overlapping maritime borders are resolved by drawing the various buffer zones out from the shoreline of all coastal land of each country, and where they conflict take the approximate midpoint of the overlapping area as the border.

Small islands can therefore have an outsized impact on extending the territorial waters of a country, which is why there are so many territorial disputes over seemingly-insignificant uninhabited islands.

There's not always an unambiguous resolution to competing claims, see for example:

- China's "nine-dash line" https://en.wikipedia.org/wiki/Nine-dash_line

- The Israeli–Lebanese maritime border dispute (which was only resolved last year) https://en.wikipedia.org/wiki/Israeli%E2%80%93Lebanese_marit...