I'll add that : If  "In the early Church, each city had one overseer or pastor" ..it seems that eventually the 'city' became a 'territory' or 'Diocese' and the Bishop delegated people to 'extend his presence' to sub-divisions or groups of people we call 'parishes'.

That's all just practical custom
s that were found to 'work'. Nothing 'Doctrinal' about those arrangements until you get into the concept of the 'Sacrament' of 'Orders".