pasting HTML to page

Using the CSS online school examples, I figured out how and where to post my code from my other website so that it looks just as desired. I put part in the body and head, so it looks as in the screen shot, below the example’s ‘This is a paragraph’ See https://theearlybirder.com/species%20list%20in%20CSS%20school.JPG

So then I went to Elementor and using an HTML tag, just pasted the body part in, and it is useless with huge spaces without links and so it needs the header part. But I am afraid to paste there as maybe it will affect the whole site. How do I see the whole WP page code so I can figure it out, or perhaps someone can advise.

The page I need help with: [log in to see the link]