How does Google Analytics determine unique visitors

Website operators who start with web analysis usually first want to know how many visitors a website has. In order to gain knowledge about this, two measured values ​​are of particular importance in Google Analytics:

For analytics newbies, there is initially no discernible difference between these two measurements. Aren't the number of sessions and the number of users the same? A look at Google's support database provides clarity:

Definition of "meetings":
A session is a group of interactions that take place on your website over a period of time. For example, a single session can include many screen or page views, events, social interactions, and e-commerce transactions. SOURCE: Google

Definition of "user":
The measured values ​​Users and Active users indicate how many users have accessed or interacted with your website or app. SOURCE: Google

So it turns out that sessions and users have to be fundamentally different. The interpretation of this data becomes easier when the technology behind the analysis is understood.
Here are a few key figures:

  • A session lasts at least 30 minutes (session terminated if inactive) and is extended accordingly with each interaction.
  • Each session ends at the end of the day at midnight at the latest
  • A user who leaves the website and comes back to the website via another campaign creates a new session

So it is important to know that a User multiple sessions can generate. This is the case, for example, if the user visits the same website several times a day. It is therefore of course the case that the number of sessions must inevitably be at least as large as the number of users: