Php guide for WordPress

Dear staff,
is there a guide to understand how exactly php works in wordpress?
I’d like to understand how the data we put in a post/page (which are in a database) are displayed in the site. I know that page.php and post.php is used to define how a post/page appears, but I don’t know how wordpress can pick up the data from the database and put them in a post/page according to the templates defined by page.php and post.php. Probably there’s another php document that does that, but I don’t know how the all thing works. Or maybe the page.php/post.php do that just by themselves.
Please let me know if there’s a guide which explains this kind of things.
Regards