updating content

okay, this might be a not very smart question – but after looking at all these php files, where does the content actually come from? Is the only way to add the actually content say of the body section -thru the dashboard/admin area? or can I add this into it’s own separate file-html/php – too?

I saw that bloginfo() for example can pull stylesheets-in a header – but it’s grabbing one of the css files…
But what about the bloginfo() for the title – where does it grab this title from? Or the content area – how do/where do you pull the info for the the_content or the wp_link_pages for example?? I’d rather do it all through code than logging into as admin and updating through the dashboard. is thsi possible??
thanks