If I submitted my sitemap into google for my blog, does that mean that google knows exactly when I update my blog?
no...not at all. Google uses sitemap to know about all the URLs that are present on your website. To inform Google about the update you need to submt new sitemap everytime which is a bit tedious process. If you are using WP or Blogspot then you can install a plugin which will automatically update your sitemap when a new URL is added
Most blogging software, such as Wordpress or blogger, will automatically send a notification (ping) to Google blog search letting them know when a post has been published. A sitemap.xml file will not guarantee that Google knows when your published posts. It will provide a guideline for Google on when to come back and find new content on your site.