How are rewrite rules applied in WordPress?

Question

How are rewrite rules applied in WordPress? I don't see any rules in the .htaccess file...

For example, if I set the address URL to www.example.com, then calls to example.com will 301 redirect to www.example.com, but I don't see any rule for that in the .htaccess:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress 

Answer 1

index.php acts as a router. All requests are sent through index.php which looks up the rules in the database and directs you to the correct location. This should give you an idea of how a PHP router works. The .htaccess just removes index.php from the URL. More rules can be created using the WP_Rewrite class.

Answer 2

When Your index.php is loaded wordpress askes for your "website url"
That adress is preDefined in your database.

As for posts and pages in the case you are using pretty links meanina
a custom permalink structure & in the case you have changed the url after initial draft is saved wordpress uses a 301 permenent redirect.

If you are asking from an seo point of view you should know the your Base URL is the one you have set in your admin meaning:
settings >> general >> WordPress address (URL)

So... (from an seo point of view) you should take a moment to set
up the preffered address in google webmaster tool in order to keep all your link juice directed to the right place since technicily speaking
somewebsite.com & www.somewebsite.com are 2 different addresses.