A sitemap is a list of all webpages of a website accessible to crawler and user.
HTML Sitemap is build for website user to easily access all web pages.
XML sitemap is build for crawler to easily crawl all web pages.
Crawler or Webspider does not read sitmap in html, .txt or other format, it is read sitmap only in .xml format.
When you create a sitmap you basically gives a blueprint of your website to google so with the help of this blueprint google can easily find out which page is linked with which page but if you not provide the sitmap then the google got confused to find out the relation between the different pages .
You can easily create sitmap by using yoast Seo. You just need to install the yoast Seo plugin and then click on the general option and in general option you can see the features option and when you go down below in features option you can see there is a text like this (XML sitmaps ?).You simply need to click on that question mark and it show you the link(See the xml sitmap)click on that link and it will show you the sitmap of your website.
XML SITMAP CODE
We can submit XML Sitmap in different-different Search engine’s Webmaster for fast indexing.