Help creating system-wide overlay

Hi!

I'm planning to make an overlay to display different information of my computer's status in an HUD that sits over any game. Just like Oculus' Performace HUD (https://developer.oculus.com/documentation/pcsdk/latest/concepts/dg-hud/).

The idea is to get it working over every game/app (even in Oculus Home if possible) without the developers having to do any integration in their game.
I know this is possible because of this: http://www.hellov.io/, but I don't really know how they did it or where to start.

Can anyone point me in the right direction?
Thanks!