How to determine the session of an individual user?

Question

PHP

Each user has his own session ID and his own data in sessions.
Session_start(); is used to find the ID of an existing session, but how does this function determine which user session is being used?

Answer 1

From the user's browser, an identifier is transmitted in the request, by which the user is determined. session_start() only opens the session, but any access to session variables already returns their value by user ID.