Why is my device a touchpad and a mouse and a keyboard?

If you have spent any time around HID devices under Linux (for example if you are an avid mouse, touchpad or keyboard user) then you may have noticed that your single physical device actually shows up as multiple device nodes (for free! and nothing happens for free these days!). If you haven’t noticed this, run libinput record and you may be part of the lucky roughly 50% who get free extra event nodes.
↫ Peter Hutterer

I’ve honestly always wondered about this, since some of my laptops shows both a trackpad and a mouse configuration panel even when there’s no mouse plugged in. Thanks to this article, I now know why this happens.