ViewState is basically encrypted and incorporated as part of 1 hidden field. IT is nothing like a session variable.

The viewstate  of all controls on page  and your own viewstate gets all combined into one hidden field and packed/unpacked transparently.

Look at:
www.learnasp.com/.../pageeventmodel.aspx

Now run the page, i.e.
www.learnasp.com/.../textbox.aspx
several times and enter values and push the buttons. Notice how it always remembers the 3 textbox entries. It could remember the contents of any runat="server" controls, inside form runat="server" tags and anything you manually add to Viewstate.

View the HTML source. Notice the =>
input type="hidden" name="__VIEWSTATE" value="dDwtNDE...">
is how this moves around.

Unlike session state which lives on a server  and consumes memory ViewState is just a HTML hidden field. You could have the browser open and push that button 3 days later and that hidden field mould still do its job with no cost in server RAM.