/page and /page.html are different web pages and technically different URLs but often many websites contain the same content for /page and /page.html links. Web servers are configured in such a way that if you open one of them, you are automatically redirected to the other with the same content.
Since they are technically different pages, however, if /page and /page.html contain different content from each other, Google can index and rank them separately for different keyword results. Gary Illyes from Google confirmed this in a tweet saying that they can index and rank them differently if they have substantially different content.
Sure, we can index both of those, if they have substantially different content
— Gary “鯨理” Illyes (@methode) June 26, 2017
It is recommended, however, to create a different URL for different content and not get into this fiasco of creating a similar /page and /page.html links for different contents. As is conventional, one should try to redirect one link to the other instead of making different contents for these URLs.