Why do smartphones get hot when you are just scrolling

My phone gets noticeably warm after using it for a while even if I am only scrolling social media or reading articles. It does not feel like I am doing anything intensive at all yet the back of the device still heats up.

Is this completely normal for every smartphone or could it mean the battery or processor is starting to have issues? Is there a simple way to keep the phone cooler during everyday use without closing apps constantly?