Multiple Users Environment

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C#

ASP.NET

I want to know about development of multiple user environment websites like Code Project, Facebook,Yahoo etc........
I want to know how it works ....like in these websites millions of users are joined.
And each user has its own pics , comments ,posts like each has its working environmment..

I am new in development And still I learn that to get any data queries are used with some conditions...But how it is done in multiple user environment ....Is there queries for comments ,posts etc.. with userId's ..Such type of questions are confused me ....Please some body guide me all about it.