XML Sitemap file contains every post, page and sometimes categories, images etc from your site/blog. It helps search engines to better indexing your site.
Blogger does not provide this file, like wordpress does for example. So, you must do some work for yourself, in order to have a full list of your content and not the last 26 articles, by default.
So, if you have a blog on blogger, you can do this simple trickQ
Go to this page and type your blog's URL. Then, you will get a text where you have to copy it and paste it in robots.txt settings (go to Settings > Search preferences).
Now your blog is ready, search engines will starting to crawling and indexing your content.