How to load a page using ajax?

Question

JavaScript

Good evening!
I am making this page . At the moment, I did the following
In the original, the pages are loaded using ajax, as far as I understand, I
almost did not work with ajax. I have read the documentation, but I have no idea how to implement it yet. I see the page structure almost does not change. Only the background image changes, the svg itself and some styles. It's not clear to me how to load it with ajax to the right places.
Tell me your ideas or where you can dig