I want to show animation while page transitions with vanilla javacsript

I have been lookind for javascript eventlistener and all to see if i can do normal page transition animation when i click on a link to visit another page.

visit: https://pelizzari.com/en/

Visit this site and you will see that if you click on any of the project it does this sliding animation, the screen doesn't get whitish and then load. When i click on the link it shows animation and then immediately shows the page.

I want to do this with VANILLA JAVASCRIPT. I am trying to learn javascript very much thoroughly. That's why I have been looking for ways. And also how to do the slow scrolling. In the site, after clicking on any project in that page, when i scroll down it is slow and smooth. how do i do that??