Why is there .html in my blog post

Have you manually added the extension in your Permalinks settings ??
https://wpmudev.com/blog/wp-content/uploads/2011/12/html-on-url2.jpg

Do you have some sort of Permalink plugin installed?
https://wordpress.org/plugins/add-html-extension-to-pages/

Do you have any re-write rules set in your .htaccess file, or in your functions.php file?
https://toolset.com/forums/topic/remove-slug-and-add-html-extension-to-url/

I’d imagine there is some regex you can add to your htaccess file in order to re-write the HTML from the page.
Or you could use a Redirection plugin and input all your pages to 301.

Or – you could just leave it alone?