a method of performing a page transition by leaving only certain elements

I would like you to look at this site first.
→http://kekselias.com/

If you click any of them in the left navigation, the main element slides to the right and disappears.
The following elements slide from the left:

What I don't understand in this animation is that even though url is different, the left header is
It's always on the screen.

I'd like to know how to change pages with url, but leave only headers (specific elements) to transition pages.

Please let me know if anyone understands.

I can only write a little bit of HTML, CSS, and jQuery, so
I would appreciate it if you could explain it without using too difficult technical terms.

Thank you for your cooperationm(__)m