How & where multiple instances of objects are stored in web application

Question

As in Web application multiple users work simultaneously and each user has their own copy of data so how application managed this. Whether different user's data are stored n different heap/stack. As HTTP is a stateless protocol and session maintain every users data so is session is pointing to a block of data that is stored for that users based on activities he/she has doing on web application or something else.

I guess my question is un-clear so giving some example. If in web application there is a class userdetails

public class clsuserdetails 
{
     public void AddUserDetails(string UserInformation)
     {

     }
}

private void Button1_Click()
{
     clsuserdetails objuserdetails = new clsuserdetails();

      objuserdetails.AddUserDetails(txnuserinformation.text)
}

In this scenario as multiple user can click to this button1 (For ex. 1000) so whether object objuserdetails will get stored in same heap or different heaps are created for difference user (1000 heaps in this case).