On Android, it's probably a little utility software called Quick Cursor (it's not FOSS). It's incredibly convenient being able to spawn a cursor on your phone from thin air that you can use to reach the "unreachable" portions of your screen, especially if you are holding your phone with one hand. Besides being a "phone touchpad" it has a bunch of ways of triggering actions/shortcuts, for example: volume or brightness control, launching an app (I use it for launching a floating calculator, notes...), opening notification shade, copying text (it can copy any text that is under the cursor, even if it's not selectable)...

It's not that I couldn't go without it, but it changed the way I use my phone and it would feel really weird without it. It feels like it should be a part of the OS.