Where can I find information about how GA collects user data?

Question

Google Analytics

Dear, I need your help in a delicate matter. The task can be described as follows: It is
necessary to describe the logic of work for building a system of end-to-end analytics by analogy with the work of GA. Or rather, its part of collecting and storing data about visitors and buyers.

• We need to identify the source of the user's appearance on the site,
  
• Glue the source with the user id in the database
  
• Glue order data with a specific user and session.
  

If someone can advise where on this topic to read the documentation or cases. I would be extremely grateful.