C++ Graphs (Logic)

Please do not post links, code, algorithms, or copy another reference unless directly related to my question. I am looking to understand a certain aspect, and this aspect only.

I am trying to fully understand graphs in C++. I am trying to understand how to get the shortest distance from point A in a graph to point B. However, let's say our graph is a graph of a city, or country, or bigger. Now if we want to get the shortest distance from point A to point B we have a problem. The problem is that there are billions of streets and different ways to go and the user wants a response quickly, what can we do?

Do graphs have to be created in a way that basic assumptions can be made? For example south-west to north-east, would you want to assume north over south west over east?

Can you ever get THE shortest way or do you have to settle?

I think from this you can understand where my confusion lies, I do not require specific answers to specific questions, simply see my confusion and guide me if you can, please.