How are changes to the site submitted to the sitemap and then google?

[support23]

I just checked the sitemap I installed a year ago and realised it has not been updated for a year! 

In that time I have changed hosts and the generator folder had dissapeared, so I deleted the old sitemap and did a fresh install. All is working.

What I want to know is that does the sitemap.xml automatically update itself when a new link is added on the website or do I need to do something everytime a change is made?

In turn, does google/yahoo automatically crawl the sitemap after a change?

I have the configuration set up as:

Change frequency: Daily
Last modification: Use server's response

[XML-Sitemaps Support]

Hello,

in order to automatically update sitemap you should setup a scheduled task (cron job) in your hosting control panel.

Sitemap generator automatically pings search engines to notify them about sitemap update.