How to implement such a transition between HTML pages?

Question

HTML

www.tramont.in
On this site, when you open the menu and go to the "Who I am" page, there is animation, and no AJAX is used. That is, the transition between two HTML pages is animated. How can this be implemented?
Thanks in advance)

Answer 1

AJAX is used - on the first click, a request is made to www.tramont.in/about.html and then this block is cached.
JS shows the required block, the address bar is changed using the History API