how to slow down the speed of scrolling my website

i want to make scrolling and moving trought my onepage website slower,

i have tried several questions like plugin Nicescroll but non of it worked.

I am using
.html {scroll-behavior: smooth;}

but i would like to make whole scrolling and moving throught buttons slower like this here is an example https://layouts.b3multimedia.ie/free-divi-layouts/finance/

any ideas ?