Whenever you select a country for search, the city with the highest population in that country is set as a default.
By clicking on the City box and starting to type the name of a city, you will get an autocomplete of supported cities.
Clicking on one of the cities in the drop down will select that city for your search.
Will be result be localised to that city?
Sometimes. Google determines when a search query has enough local intent to show localised results. This means sometimes you can change the city, repeat a search and the same results.
Can I do a search without a city?
No. There is no such thing as a "national" search in Google. Any regular search you would perform on Google would always pass location information to Google, so we have to pass something. Previously, before we had the city customisation we would simply pass the location of the largest city in the country in the background.
What does it mean if the results are the same for two cities?
This is helpful information that tells you there is no real difference in search intent for searchers in these two locations. While it does not necessarily mean you don't need another location page, it tells you the intent is the same.
What does it mean if the results are different for two cities?
This informs you that the searchers in the different locations has different priorities and are perhaps interested in different things. You should consider this when you produce content ofr both areas.