How is the animation implemented when scrolling to an element on the site?

Question

Animation

Tell me, please, how is such an animation implemented, when, upon reaching a certain position on the site, does it start to work? What technologies are used? How to do for yourself?
Example: sell.fairandsquare.ru

Answer 1

mynameismatthieu.com/WOW

Answer 2

function isScrolledIntoView(elem)
{
    var docViewTop = $(window).scrollTop();
    var docViewBottom = docViewTop + $(window).height();

    var elemTop = $(elem).offset().top;
    var elemBottom = elemTop + $(elem).height();

    return ((elemBottom <= docViewBottom) && (elemTop >= docViewTop));
}