In 1997, as part of learning the Postgres backend source code, I created an image of the backend source code flow and added hot-links to the image so users could click on a section and get detailed information, including a link to the actual source code. This was on the Postgres website for many years, but because of website changes and the switch to git, it was removed.
After recent requests, I have redone the flowchart and moved it out of our source tree; it now exists on the main Postgres website, linked to from the Developers/Coding section. The detail text has been moved to a Postgres wiki page, to allow for easier community improvements.
So, if you were ever curious about how Postgres works, head on over to that web page and start clicking.