This is the default template without a featured image
If you include an excerpt via the document tab, your post will display that text in the header under the post title. I made an example post with an excerpt. Posts also display the category, author and post date in the header.
Pages do not have the excerpt option. Only your post title will appear in the header section when you use the default template in twenty twenty theme. The page you are looking at is a page without a featured image, so only the page title displays in the header.
You can set a featured image, via the document settings tab. That featured image will be centered, or sit on, the dividing line between the header and body sections of the page. See this example of a default page template with a featured image.