A somewhat recent trend I've noticed is web pages including a "scroll down" icon, to signify that the bulk of the content is "below the fold" of the page. There are two main variants that I've seen in the wild. One being a simple, usually bouncing arrow, and the other being a mouse icon with a scroll wheel.
(Examples taken from: https://codepen.io/nxworld/full/OyRrGy)
I personally always found the mouse icon confusing, and to be honest, I thought it was supposed to resemble a scroll wheel. (I'm still not entirely sure)
I also realize the scroll wheel icon wouldn't make sense on mobile devices. Instead, it could be replaced by a "swipe" icon.
What is the best way to notify a user that scrolling down is required to see the main content? And how should this be treated on mobile devices?