How Does WordPress Handle Upgrades?

Can anyone describe the pseudocode for how WordPress handles its upgrades? As in, you go into WordPress admin and choose to upgrade the version of WordPress.

I mean, does it use FTP APIs locally? Does it send credentials to another host which reconnects back with FTP APIs? Does it download files with Curl? Does it polyfill if a given API isn't there and go 开发者_JAVA技巧another route? Does it test file permissions to see which API to use?

I've got a client who wants something like this built into a web application unrelated to WordPress.

---

Have a look it wp-admin/includes/update.php and wp-admin/includes/class-wp-upgrader.php

And here for some explanation: http://tech.ipstenu.org/2011/how-the-wordpress-upgrade-works/