The general technique I can identify is that the webpage uses an approach called "scrollytelling" (also noted by the references to "scrolly" from the Web Inspector) to make the animation advance in reaction to the user scrolling down.

For the pictured objects, I assume that an animation team (likely a bit separate from the web development team) created the assets. But I wonder how the developers then made sure that each object enlarged at the proper rate as the user scrolled down (and also made sure that the scaling worked properly upon changing the width of the window).