How to determine the source (views/modules/blocks, etc) of content in different URLs?
If you are comfortable looking through your database directly, the quickest way to find out where $content is coming from is to look at the menu_router table and reference the path and page_callback fields.  For example, path node is generated (normally) by node_page_default and it creates the default front page.  Looking through this table you can see where all the other paths on your system are referenced.

As for where all the other content comes from, eg, all the blocks in other regions of the page, there you have to look around through the blocks table and look at the various information there, eg, module and delta say which module creates the block and which one it is, region says where on the page it will be put, and pages have any restrictions as to what pages the core block module knows about.