Hello.
XML sitemap is just an xml file where you define the structure of your webpages. How many webpages are there and how can one reach there. These sitemaps are designed for crawlers.
There are two ways to create a sitemap. First, one is the manual way where you write down details about each and every web page. The second one is using software, either online or offline. An online search for "XML sitemap generator" will give you plenty of options to start from. Many CMS software come with an in-built sitemap generator. But, they are often not optimized.
Hope it helps
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